Re: Why did spock end up in the alternate universe??
The in-universe reason why they don't do it is because they have the Temporal Prime Directive, Temporal Investigators and in later centuries Timeships that undo violations of the timeline.
Nobody would be allowed to stop Sisko from travelling to the Gamma Quadrant, because that was simply the natural flow of things.
And nobody would allow Nero to fuck up the past over a period of 25 years and kill 6 billion people. Somebody on one of the timeships would notice (since this timeships are unaffected by the changes), and prevent Nero from coming out of the black hole in front of the Kelvin and destroying her.
If NuOldSpock is indeed the Spock from the original TOS universe (which is still debatable as far as I'm concerned), then he had access to numerous methods of time travel at his disposal. He would have knowledge of these methods having used many of them in the original universe....My point is that the Spock we know from the TOS universe would more than likely have at least made some effort to try to figure out a way to go back and save the six billion inhabitants of his home planet (if he did indeed believe that NuVulcan was "his" home planet).
That's true of every disaster that's ever been in Star Trek. Every time something has gone horribly wrong, somebody could take a ship, fly around a star and go back and fix it: tell Commodore Decker to stay out of the way of that planet killer, divert the USS Intrepid away from the giant space amoeba, warn Sisko and company to stay out of the Gamma quadrant, and so on. The only reason they don't is because then nobody could ever tell interesting stories again. If we accept easy time travel in the Star Trek universe, we also have to accept that characters will forget, based on the needs of the story, that they can hop into the past any time they feel like it. The new film is no different in that regard.
The in-universe reason why they don't do it is because they have the Temporal Prime Directive, Temporal Investigators and in later centuries Timeships that undo violations of the timeline.
Nobody would be allowed to stop Sisko from travelling to the Gamma Quadrant, because that was simply the natural flow of things.
And nobody would allow Nero to fuck up the past over a period of 25 years and kill 6 billion people. Somebody on one of the timeships would notice (since this timeships are unaffected by the changes), and prevent Nero from coming out of the black hole in front of the Kelvin and destroying her.