Allow me to illustrate. Let's say it happens as you say, Steve goes back in time and he and Peggy carefully hide his existence so that the rest of the MCU proceeds as we have seen it proceeding, and Old Steve strolls back onto base after taking the long way home. First of all, Old Steve is way too young. But secondly, this is what would happen:
Sure, it seemed like a good idea at first. “I’ll change my name and stay home all the time,” he said. “I won’t interfere in ANYTHING that’s meant to happen, so eventually, I’ll get back to when I was supposed to come back, and we’ll all live happily ever after!”
Until the day Peggy comes home and starts telling him all about how she and Howard are starting a new organization to keep up the work of the S.S.R., and they’re going to call it SHIELD, and they’re recruiting all these experts to—
“Wait!” Steve interrupts, suddenly panicking. “There’s something I have to tell you about the people you’re recruiting for SHIELD! Some of them—”
“No, no, no you don’t,” she says quickly, standing up and leaving the room so as not to let him continue. “You cannot interfere in anything, remember?”
Steve sighs. Yes, he knows. But it really bothers him that his wife will be working with HYDRA.
Then they’re listening to the news. Vietnam War, complete mess— no, he wouldn’t even make it that far! KOREAN War. People in danger. “I can help! I need to go help them!”
“NO YOU DO NOT,” Peggy says, switching off the radio. She now will not let him listen to the radio. Or read the paper. They don’t even get a TV.
Steve’s getting antsier. Things are HAPPENING out there. Obviously things are happening because Peggy still works ridiculous hours and occasionally looks very worried. How can he sit here at home letting people suffer, all in the name of living “happily ever after”? How could he be so selfish?
Finally, he snaps. “Peggy, HYDRA have infiltrated SHIELD and we need to weed them out and then I’m going to become a secret vigilante so I can help people without letting them know I know the future!”
Peggy stares agape at him. He continues, “I can’t do it anymore. We just started a new timeline, and THIS timeline WILL be happily ever after. In a few years you need to hire a fellow named Hank Pym, and he’ll help me get back to the others eventually, just so I can pass the shield onto Sam. It’ll all be great!”
Zap. The entire MCU is wiped out.
Well, no, not really, he just made a new timeline!
But I’ll humor you all who want to believe this all happened in the same timeline. You tell me, okay, he went back to AFTER the events of Agent Carter, so Peggy’s star turn in the MCU doesn’t get wiped from existence, and all the rest of the movies in the MCU can still have happened in the timeline because she’s really good at keeping him a secret, and why WOULDN’T Steve be good with it because he knows the importance of letting things happen? The MCU remains intact.
Well, let’s try it:
Steve Rogers shows up in the NY offices of the S.S.R, 1947. Things appear to be in chaos there because apparently, he gleans from the chatter around him, the office chief was just found shot or something. “Excuse me,” he says to someone he catches hustling by, “Could you please tell me where I could find Agent Margaret Carter?”
The man looks surprised. “Peggy? Oh, she’s out at the L.A. office. Just put in a transfer notice that she’s staying there ‘indefinitely’.”
“L.A.?” Steve makes a face. “First the Dodgers, now Peggy? What the hell is wrong with Brooklyn?”
“Beats me,” says the man. “Thing with Peggy, well,” he shrugged, “there was always kind of a thing between her and the West Coast Chief, know what I mean? Least we all figure that’s why she’s staying.”
“Oh,” Steve’s face falls. “Of course there was. Why would she wait around for an icicle? I knew this was a bad idea.” He pushes a button and disappears in front of the startled agent’s eyes. Blip.
Oh, like, some rando is going to gossip about Peggy’s love life to some stranger who showed up in the office, you say. Good point.
“…First the Dodgers, now Peggy? What the hell is wrong with Brooklyn?”
“What’s L.A. got to do with the Dodgers?”
“Nothing! Yet. So… could I have her L.A. contact information?”
The man narrows his eyes. “Who are you, anyway? Are you authorized for that information? Wait a second, how’d you even get past the switchboard girls?”
[Record scratch. Rewind]
Steve Rogers arrives in the front room of the NY Bell Telephone offices, 1947. He meets the smile of the switchboard operator nearest the door. “Excuse me,” he asks her. “Could you please let me in to see Miss Margaret Carter?”
The woman’s smile turns apologetic. “I’m sorry, sir, Miss Carter doesn’t work here anymore. She’s moved to L.A.”
“L.A.?” Steve makes a face. “First the Dodgers, now Peggy? What the h—” he clears his throat. “I beg your pardon. Could you, please, give me the address of the L.A. office?”
Now the woman’s smile turns awkward and forced. “The L.A. office of Bell Telephone?”
“Uh, right, no. Listen,” he lowers his voice, “I know this is an S.S.R. office. I’m a friend of Peggy’s from the war. I have no other way to get in touch with her, so if I could just—”
“What did you say your name was again, sir?” The woman bats her eyelashes while covertly pressing a button beneath the desk.
Steve catches the furtive movement. “Uh, I’m… ah, sh—I knew this was a bad idea. You know what? Forget you ever saw me.” He pushes a button and disappears in front of the startled operator’s eyes. Blip.
OH COME ON, at least have him try to find Peggy in L.A!
Okay, okay then.
“What did you say your name was again, sir?” The woman bats her eyelashes while covertly pressing a button beneath the desk.
Steve catches the furtive movement. “Uh, I’m… is Howard Stark here, by any chance? He’ll know who I—”
“I’m sorry, sir.” Her smile turns outright incredulous. “Would you like to speak with my supervisor? He’s on his way now.”
At that moment a door opens at the far end of the bank of switchboards, and an older gentleman steps out.
“Hello, there, I’m Vernon Masters. I’ve taken over here during this period of, uh, transition.” The man holds out his hand, glaring through narrowed eyelids at Steve. Steve takes the proffered hand and gives it a firm shake. He doesn’t trust the guy, but doesn’t see another option. “I understand you’re looking for Miss Carter?”
“Agent Carter, yes.”
“I’ll call the L.A. office and let you know if they’ll let you speak with her. Who should I tell them is calling?”
Steve sighs. Why is it so hard to covertly track down professional spies? It’s like they don’t want to be found or something.
Okay, for the sake of argument, let’s just say Masters gives him the address. I don’t know why. He’s a sleazeball and thinks it’s funny to give out top secret info to complete strangers? Yeah, probably.
Steve puts the address in his pocket and pauses. “Can I…borrow a plane?”
Masters laughs. “Yeah, right. No. Good luck, son.”
So, somehow or another Steve scrounges up money for a plane ticket, or he hitchhikes, or he harnesses the remaining Pym particles to make a space jump without a time jump, I don’t know, but we’ll conveniently skip over that part…
…and arrive in the lobby of the Auerbach Theatrical Agency, still 1947, where a bespectacled redhead is shooing a disgruntled-looking tap dancer out the door. She dusts her hands and notices Steve. “Please don’t tell me you juggle swords.”
“Uh, no. I, uh, can’t say I ever have.” He clears his throat. “Actually, I’m looking for Miss, uh, Agent Margaret Carter. I was told she left the New York office for this one.”
“AH.” The receptionist reaches for the phone on her desk. “Who should I tell her is here?”
Steve lets out his breath and smiles. She’s here. He could maybe be a little less cagey. “Just tell her it’s… Steve.”
The receptionist freezes, staring at him, and the receiver slowly drops from her hand. “NO. You can’t be. You are. It is you!”
Steve feels himself blush. “Try to keep it on the down low, all right?” he murmurs.
“All right?” she repeats, picking up the receiver again and pushing the call button. “Hey, Chief? Is Peg with you? She’s got a—” She breaks off, her smile suddenly falling. “Oh golly…”
“Rose?” a man’s voice crackles out of the other end of the phone. “You okay?”
The receptionist giggles weakly. “Yep, Chief. Everything’s, um, fine. Someone is here to see Peg.”
“What’s wrong, Rose?” the man’s voice begins again, but it’s interrupted by a crisp Englishwoman’s voice, and Steve’s heart leaps. “Did she say someone wants to see me?”
“Come right on down!” Rose, the receptionist, says cheerfully, then drops the receiver back into the cradle. She turns a forced smile toward Steve. “Go on and have a seat while you’re waiting.” She turns away muttering something that sounds a lot like, “so AWKward!” through gritted teeth.
Steve frowns. “Am I… intruding on any—” he begins, but a sound of rushed footsteps and even more rushed conversation from the top of a nearby stairwell attracts his attention.
“…she just sounded worried—” says the man’s voice from the phone, quickly drowned out by, “Whoever it is, I’m sure I can handle it, Daniel, yes, especially if it’s Dottie, you know she only listens to…”
And there she is. She stands, gawking, at the bottom of the stairs. Dark curls framing her stunned…stunning…stunned AND stunning… face. The bright color of her dress brightening the colors of her face—he’s never seen her in bright colors before.
A dark-haired man shifting his weight between the stair railing and a metal crutch hobbles in behind her, eyeing her sideways as he catches up. “Peg?”
“STEVE!” she exclaims at last. Her mouth opens and closes and opens silently a few times before a sound squeaks out again. “Where have you…you were…we looked for you…you were lost at sea, we thought…” she trails off into silent gawking again. The man behind her looks suddenly horrified.
One part of Steve’s mind registers this, but most of it can see only Peggy. “I know. It’s—a really long story. But I’m back now! And I wondered if you were still interested in that dance.”
Peggy continues to gawk, shakes her head as if to clear it, says, “I…” and blinks. There is silence for another long moment.
“Captain.” The man with the crutch shuffles past Peggy and holds a hand solemnly out to Steve. “Corporal Daniel Sousa, 99th Infantry. I owe you my life. We were freezing and starving behind German lines and you got us out. I wouldn’t be here today without you.”
“Pleased to meet you, Corporal.” Steve shakes his hand, notices that his eyes drop quickly. Steve frowns.
Sousa turns back to Peggy with an air of complete dejection. “I…I guess I’ll see you at work tomorrow, Peg.” He hobbles toward the stairs with his head bowed, but Peggy snaps out of her shocked stare at Steve and spins toward him.
“What’s that supposed to mean?!” she demands.
Sousa looks up, a touch of puzzlement crossing over the dejection. “Take the rest of the day off.” He nods toward Steve. “Your Captain is here. We all know what that means.” He starts to turn toward the stairs again but Peggy grabs him by the shoulder.
“Are you seriously insinuating that I intend to dump you? Are you dumping me because you think I intend to dump you?”
“Wait a minute,” Steve interjects, feeling suddenly sick, but Peggy roars on.
“You do realize you are doing to me exactly what Violet did to you?”
“And she was right!” Sousa retorts.
“STOP!” Steve barks. Everyone freezes. “Now just so I’m clear on this… you two are… together?”
“Yes,” Peggy says firmly, at the same instant Sousa says, “No.”
He continues, “I mean we were. But we thought you were dead, and, you’re not, so obviously—”
“Not ‘obviously,’ Daniel, how DARE you presume to make my choices for me before I’ve even had a chance to make my choices for myself!”
“OKAY, you’re right, I’m sorry!” Sousa backs away from his fire-breathing girlfriend. “YOUR choice. What…what is your choice?”
“My..choice.” Peggy blinks and looks back and forth between the two men. “My choice… is that this is a positively absurd situation and there’s a murderer on the loose and I’ve got to get right back to tracking her down and no one should attempt to distract me!” She marches toward the stairs.
“Peggy, wait!” Steve calls somewhat weakly, but she hears, and stops. “You don’t have to choose. It’s obvious. I’m the one who doesn’t belong here. I made a mistake. I knew it was a mistake. And now I’m going back to my own timeline. Take care.” He reaches for the button, and…
[Pause. No, *I* can’t leave this here. We’re neglecting some people!]
…two men burst through the door. The shorter of the two says, “Great! You’re all here, listen, I had this crazy idea for an anti-matter engine and—”
“Mr. Stark, this is a really bad time—” Rose lunges up from the desk to hold him back, but he’s already frozen in place.
“Holy Bejeebus.” He turns white and sways off his feet. Before he topples over, the taller man catches him and whispers, “Easy, sir.”
“Is it…am I…are you…?”
“Yes, Howard,” Peggy says softly, “it’s really him.”
Steve smiles. “Howard. It’s good to see you. I forgot how much you changed as you got older.”
“Oh, god, Rogers, I tried, I tried so hard to find you, I really did—”
“Shhh, I know, Howard.” Steve places a hand on Stark’s shoulder and leans in. “It’s all right.”
The taller man peers astutely between the two, and then remarks, in a precise British clip, “Am I correct to infer that this is the illustrious Captain Rogers whom I have heard so much about?”
Steve knows he’s never seen the man before, but something in his voice and manner is so familiar that he grins as he shakes his hand. “It’s a pleasure to finally meet you, too, Mr. Jarvis.”
“Oh!” Mr. Jarvis looks startled, then bites his lip as if about to say something more but unsure of what.
“But as I was telling everyone, I can’t stay.” Steve straightens up and addresses the room again. “I don’t belong here anymore. I have been traveling through time itself to get all the pieces back into place, and now I’m out of place. I need to get back to my own timeline. You’ll be all right without me. But maybe—before I go—some advice, knowing what I know.” This timeline is already going off the tracks, he figures, might as well set it off in a better direction. “Don’t trust Zola. No matter what he tells you. When you start recruiting for SHIELD, screen everyone for any ties to Hydra. I’m not saying people can’t change—one of the greatest agents I ever knew defected from the other side, but she’s not due for about half a century and was never Hydra, anyway so… just specifically, watch for Hydra.” To the puzzled looks around him, he adds, “It might not make any sense now, but keep it in mind.”
He turns to Stark. “Howard, try…” how to phrase this? “…remember to be kinder to your son.”
“His Daddy Issues could get pretty annoying sometimes.” He smiles through a wave of sadness. “And maybe don’t brag about me in front of him so much.”
“Jarvis, do I have a son?” Howard mutters.
Jarvis grimaces and mutters back, “Not that I’m aware of, sir.”
“Mr. Jarvis, you… just be you,” Steve continues. “You do everything right. Good work.”
“Oh.” He flusters. “Th-thank you, Captain Rogers.”
“Just Steve, please.” Steve continues to circle the room.
“Rose, I’m afraid I know nothing of your future, but if you yourself are any indication I’m sure it will be lovely.” She blushes.
“Sir.” Sousa salutes solemnly. [There is no need to read the past two sentences aloud, but maybe you should, because why pass up the chance].
“At ease; war is over, as the great John Lennon said.” Steve takes his hand in both of his and forces him to meet his eyes. He lowers his voice. “Be good to her. If you don’t, I will know.” Sousa’s eyes flash with anger. Steve chuckles. “I’m kidding. I know you two will be just fine.
“Peggy?” He looks at her, and his eyes well up. Hers are doing likewise. In a moment they are in each other’s arms. “You are an amazing woman,” he whispers. “And you are going to have an incredible life.”
“How am I supposed to lose you again, when you haven’t even given me the chance to believe you’re really here now?” She sniffles. “Why have you come, only to go?”
Why? He ponders. Because I was selfish and stupid? Because I assumed everyone would be waiting for me? Because some voice in my head said, “If you go to this moment in time you can stay secretly in your own timeline without breaking the integrity of the MCU!” whatever that means? Because… “Closure,” he says finally. “I just wanted you to know—all of you to know—” he speaks up for the rest of the room to hear, “—what happened to me. I’m okay. Frozen. Preserved. I’m going to wake up again next century and have an incredible life of my own. Don’t mourn for me. Live. I love you. I’ll see you in seventy years.”
He pushes a button and disappears before their eyes.
There is a long silence. Finally Howard says, “Does anyone else need a real stiff drink right now? I’m buying.”
For the record, after this Peggy Carter spends at least a few days being very angry with Daniel Sousa, and Daniel Sousa spends a good deal more time being insecure around Peggy Carter, which just makes Peggy Carter even more angry with Daniel Sousa, but they do, eventually, work things out and live happily ever after. More importantly, thanks to Steve’s warnings, Hydra does not get nearly as deep a grip in SHIELD as they do in certain other timelines, so on the whole Steve’s mistake isn’t too much of a blunder after all.
But you want to know what actually happened.
Well, maybe that DID happen. In yet another timeline. But regardless of how many blunders and miscalculations he might have made, setting odd timelines branching off this way and that, we do know
Steve Rogers arrives at the London S.S.R. base, 1945. He’s been missing only a week or so at this point: no one will have given up on him yet, no one will have moved on. He’ll be able to truly pick up where he left off.
And it’s late enough in the evening when he gets there that the only two people around are the only two people he really cares to see there. “So,” he says from the doorway, “about that dance….”
Peggy and Howard both turn and cry out in unison. There’s a cacophony of interrobang-punctuated sentence fragments as they rush toward him.
He waves them quiet, but can’t help laughing. “Shh, I don’t want the rest of the S.S.R. to know I’m back. This is supposed to be my retirement, see? They wouldn’t let me retire.”
Regardless, when Peggy reaches him, he scoops her up and spins her around, which only causes her to squeal, “for god’s sake, Captain!”
“Retirement?” Howard scoffs. “Granted, you look like you aged a decade in a week, but still—”
“What happened to you?” Peggy cups his face in her hands to study the changes written there.
“It’s a… very long story,” Steve says. “Almost eighty years long. I only remember a few of those years.”
“Are you talking about… time travel?” Howard’s eyes flash with interest.
Steve raises a stern finger at him. “Don’t get carried away. Spacetime is a tangled mess already. Yes and no,” he continues to them both. “I took the long way forward. The Steve Rogers you said goodbye to is currently frozen under the arctic ice.”
“What?” Peggy shoots a look at the maps and sensors across the room.
“Don’t worry. They found me in the 21st century, thawed me out. I had a lot of adventures you probably won’t believe, and now—I got the chance to come home and rest. I couldn’t pass that up. You see, this is my happily-ever-after timeline. What happened in the world I know happened, there’s no changing that. But I can use my knowledge of the future to fix the future we experience, starting now. When you two—when the three of us—replace the S.S.R., we’ll do it right this time, and then I can relax into a life of peace with,” he gives Peggy a meaningful look, “whoever wants to spend it with me.”
Peggy’s mouth twitches in something like a smirk.
“I’ll have to rejoin my friends in my original timeline eventually. There are things I’ve left undone there, and my buddy Sam will need to be trained up in the ways of Captain America. But I’ll take my time. I’ve got all the time in the quantum realm. Besides, now I have to wait until Pym shows up to make me some more of his particles.”
He walks over to the map laid out on the nearest table. “But before I hang up the shield just yet, we’ve got to keep this missing person tracking operation open for awhile.”
“You want to…” Howard scrunches his face, “find…yourself?”
“No. We’re going to rescue Bucky. Again.”
For the record, a few years later in this timeline, recently transferred West Coast-based S.S.R. Chief Daniel Sousa marries his lovely nurse, Violet, and lives quite happily, too. If you were to ask him what he thinks of Peggy Carter, he’ll raise his eyebrows and say, “The BOSS? She’s brilliant, yeah. Tough but fair. Why are you looking at me like that? Well, okay, we’re all a little sweet on her. Who isn’t?”