Can't agree with the last bit, not wholly. If he "tampered with the past" in the usual time travel heroics style, i.e. did some corrective moves in the Kelvinverse to negate the future threat to Romulus, he would not be destroying anything but instead would be letting these new people grow to their fullest potential. The future he would be returning to would just have slightly differently born Uhura and Chekov in its past, plus slightly different other heroes - plus no Vulcan and an extra Spock!
If he instead traveled to past first and saved Vulcan, too, then he'd be undoing some minor part of the story of his new friends. But he wouldn't be destroying any of them. And this new Kirk would probably make good despite the late start and the rebellious attitude and whatnot anyway, the new Spock would live long and prosper, etc.
You're underestimating the effect of time travel alterations on people. To a large extent, we *are* our histories (by which, I don't just mean events on the macro level, but the history of every part of us down to the quantum level). I mean, sure, at the end of BTTF, Marty is happier with "his" parents - they're familiar enough to feel like his parents, but different enough to seem like an improvement - and so we call that a win, but this isn't really being thought through by him or much of the viewing audience, because *those aren't his parents*. They're the parents of the Lone Pine Mall version of Marty that he saw hop into the DeLorean to escape the terrorists near the end of the movie.
So what, right? Well, if Marty never thinks of it, perhaps, for him, this is fine. But if he does, he's going to realize that HIS parents, the parents he left in 1985 originally, *that he claimed to love and probably did feel love for*, are at the very least separated from him forever by being on a different timeline, and possibly even wiped from existence altogether. An existential crisis and, eventually, a maddening mourning period for *everyone he ever knew before time travelling* might well be on the horizon for him.
As I said, Marty might never realize this. But SPOCK is certainly smart enough and experienced enough with time travel to know what he's dealing with. *On Star Trek*, we know from previous things shown that *some* things altered by time travel can be "put back on course" (a notion that I feel like I know to be nonsense in the real world for reasons that are entirely another story we won't get into here), but some things can't. Spock would know the difference, and I feel like the fact that he didn't seem to try conveys to us which one it is, too. (Or, maybe he DID try in events we haven't seen yet...)