The Undoing should have been a terrible thing. Logically, logistically, putting three and a half billion people back onto a planet in the blink of an eye should have been just as traumatic as removing them in the first place. The universe and the powers within it were not equipped to cope with change on such a scale, especially when the most powerful forces were fully wrapped up in enacting that change. The laws of physics should have screamed at the energy and mass required to create that many living bodies from dust, so much more difficult than merely giving entropy a boost towards the inevitable. The sheer displacement of air required should've shattered windows, changed weather patterns. Not to mention that if everyone had simply appeared where they'd disappeared from, there'd have been people standing on interstates, travelers falling from the sky, folks appearing in the rubble of collapsed buildings.
That was before one even considered the impact on society. In the five years since the Snap, many things had changed. Cities had contracted in on themselves, routing resources to their cores and moving the remaining citizens in towards the centers. Governments had unfurled plans of succession, held emergency elections to replace lost legislators, and eventually most managed to hold real elections again as well. With half the crops and livestock dead, many farms had shut down, while others adapted to feed the suddenly reduced population. The reappearance of the Snapped should've led to hysteria, starvation, and perhaps even finished off the collapse of society that had begun with the initial loss.
Only... it didn't. The first Undoing, when the Snapped had reappeared, was wished by one of the greatest scientific minds to ever live. The Reality Stone burned its way up his arm as he fought to bend it to his will and bring everyone back safely, even people who'd died as a result of Thanos and not from the Gauntlet itself. Some he could save. Some, he could not. But under his hand, the laws of physics bent like a sine curve and deposited the Snapped gently back into life.
That might have been enough. It would've been difficult, of course. Society probably would still have collapsed under the weight of too many people and too few resources, but worlds have recovered from such things before. It would've been better than everyone staying dead, certainly. But then there was another Snap, one that did more than destroy an alien army and wipe of the threat of Thanos from every universe. This Snap, after all, was made by the greatest futurist that Earth had ever seen.
This Snap was made by a futurist, not just to avenge the world, but to save it. The Snap that consumed his life was not just the culmination of a decade of battles fought and won, it was more than defeating a hated enemy. Tony Stark's Snap was a love letter to his daughter, a lifetime of all the things a father wishes for his children, condensed into a single instant. He wished her a world of plenty, with enough to go around, and the fields bloomed again, ruined buildings repaired themselves. He wished her a world of health, and the pollution levels that had begun to drop in the absence of so many people dipped further, sky and ground and water clearing. He wished her a world full of safety and love, and the hearts of people began to change.
Now changing the hearts of people is a tricky thing. It's a heady, dangerous sort of power, and the reason the Soul Stone was so tightly guarded. Most of its wielders fell swiftly to the darkness that came from owning the hearts of other sentient creatures. Even wishing for people to just be nice to each other was risky, fraught with all kinds of potential pitfalls. Tony could've ended up accidentally creating a universe of androids, bereft of free will, getting along because the had no choice. It had happened before with other wielders, though not on such a grand scale.
This time was different, though. This time the Soul Stone had not been activated by the murder of someone beloved. This time the stone was activated by the willing sacrifice of a woman with no debts to pay, a sacrifice that was a love song to the family she had found after too many years alone. Natasha did not Snap, but she did wish, and her wish, combined with Tony's wish, combined with the power of five Infinity Stones that would never be used together again... it was enough. The people who returned and the people who remained, for a while at least, were all made the best, most caring, most thoughtful, most understanding versions of themselves. A world primed for love.
It wasn't perfect. There were isolated riots in some places, governmental squabbles over who should be in change and what happened next, but there were no wars, no bloody coups, and no mass famines. Relief efforts were organized quickly and sent out to where food and medicine had been scarce even before everything had happened. Old friends and new friends met as though there had been no time lost, old families and new families integrated into big, complicated blends to the point where most governments had to make special rules for polyamorous “snap marriages.” The natural resilience of human minds seemed bolstered, so that even the most fragile were able to adapt to the changes better than anyone should've been able to expect.
It took awhile for anybody to realize what had happened. In the moment, it didn't seem too strange to the people living it, just the way that people come together in troubled times. It wasn't until later, looking back and wondering how any of it had ever worked at all, that people came to understand the true impact that Iron Man and Black Widow had made on the world they sent on without them. Morgan Stark grew up to put on an Iron Man suit she'd made herself and fight for the world her father had given her. Lilah Barton joined the new incarnation of SHIELD and took up the name Black Widow in honor of her beloved aunt. No one could ever forget what Thanos had done, or what the Avengers had done to stop him, but the grief eased with time and the people they loved moved on. It was exactly what Tony and Natasha had wished for.