The MU stories after TOS really stretched any kind of believability. You could maybe say that the universes were linked together by some unknown force, but at that point things get too far into hyper fantasy. The same rules don't apply to time travel, which is often shown to be more deterministic (or at least it was before JJ got to it). If JJ cared, he could make the TNG cast be alive here much in the same contrived way that the OS cast came together, but I doubt that he wants that. And as much as I love TNG, I'm glad that he wouldn't do that. It just seems unlikely that after such changes in the timeline that the same people could come together to create the same kids that would have even remotely the same lives. They should not be.