My mind to your mind
My thoughts to your thoughts
Parted from me and never parted
Never and always touching and touched
--Vulcan bonding ceremony
What is, and what was. Billions dead, only thousands saved. For most, that would be enough to mourn. It is enough for his father. It is enough for the young man he might have been, almost too much. To ask more would be unkind.
No, it is left to him, and him alone, to mourn what would have been.
He feels her loss like a knife to the heart: both the child he raised and the woman he loved. It is illogical to hope that the woman who would have been her mother should be among the scant few survivors of their world. And even if she were, it is...unethical to wish upon her the fate that led to Saavik's birth. The destruction of the Kelvin and resultant early revelations about the Romulans might very well mean that Thieurrull, as he found it, does not even exist in this reality. The good of the many would be better served by it not existing.
The life of one half-Vulcan, half-Romulan female should not matter so much. Even if he could somehow both guarantee and justify Saavik's existence, the probability of their relationship taking the same or even a similar course is highly unlikely. That it took the path it ultimately did defies logic enough, and even most humans he has known have looked askance at it. Did he not raise her from a child? Was he not the nearest thing she had known to a father? How, then, could he even consider joining his life to her as he did?
But for the Genesis planet, he might have agreed with them.
To be sure, the balance of power between them when she was young was tipped sharply in his favor. Had he not given his life to save the Enterprise, had his body not been revived and rejuvenated by the power of the Genesis device, it might have remained so.
Had Doctor McCoy failed to carry his katra, or had David Marcus survived, it might have been irrelevant.
Instead, it became her turn to raise him from a savage child, to teach him what it meant to be Vulcan, at least until the memory of who he had been could be returned to him. Regardless that the journey took mere days rather than years, it counterbalanced the scales. It made them equals. Or rather, it made them equals in his eyes: Saavik had never seen them as anything less.
Another truth he would not learn until much later, when she felt he was ready to accept it, was that her guidance in one particular chapter of his rewound, accelerated life had also made them parents. That their children will never be born in this universe is one more reason of billions that he mourns.
It was unsurprisingly simple to insinuate Jim Kirk into the heart of this younger, more human version of himself. All that had been required was to point Kirk in the right direction and tell him the captain's chair and his own unswerving loyalty were his to claim: the rest was merely the natural result of the force of that inimitable personality.
To reassemble the other piece of his heart...well, that would be much more difficult if not impossible. There are too many variables in play, not the least of which being that his younger self in this reality has already given his heart to Nyota Uhura, and he does not begrudge them that. It would be dishonest to say he had never thought that his relationship with that dear friend might have been very different if their lives had taken a different course: to know that he was correct is, in its own way, gratifying.
Nevertheless, there is a part of him that feels--more deeply than he would ever confess, even to her--that if he could only be sure of Saavik's existence, he could be content merely to find her and to raise her once again. Perhaps even to "give her away" to Jim's son, if that son too were born. It would be a...fitting way to close the circle.
He's grown sentimental in his old age. Jim--his Jim--would have laughed to hear such fanciful thoughts from him. But it would have been the sort of laughter that comes from sympathy and delight, not mockery. He, Spock, understood these things because Jim Kirk had been part of his life, thus he had taken care to bequeath that legacy to the man he might have been.
He cannot fail to recognize the irony that Saavik's legacy--the desire she kindled in him to reconcile the two disparate halves of their people's soul--was ultimately what robbed him of her.
For her sake he tried to save Romulus...and failed. And because of his failure, the man he will become instead will likely never try. It is nearly enough to make him suspect the universe is telling him that they were never meant to be.
Yet, threaded through the grief, he cannot be but grateful that he was privileged to walk, if only for a little while, in the world where that fate was defied.