The writers clearly decided to stray into the realm of time travel/quantum reality territory. They have said so in many interviews. Spock Prime is TOS Spock as established in the story by the writers. End of story. He is our link to the original show and the original universe. As far as going back and fixing the timeline.. People assume he had all this time.. Well, let's see, He goes through the black hole and immediately gets caputed by Nero, who then strands him on a planet so he can sit helpless and marooned while his home planet is destroyed.
Nero's first appearance at the beginning of the film sets up a new timeline/alternate reality the moment the lightning storm appears in space, and everything changes from there. How is Spock going to go into this new future and prevent Nero from going through the black hole in the first place?? This reality set up an entirely different chain of events that are no longer predictable, much as Spock explains on the bridge. All Spock Prime can really do is try to get Kirk and Spock together to fight Nero and help rebuild the Vulcan race until he can figure out some other way to put things right. This isn't rocket science, people.. It's quantum mechanics.
Kirk and Spock destroy the Narada to prevent that ship from going through another black hole and wreaking havok in another timeline, assuming Nero might survive it in the first place. Now would they take a chance and let him live to do that?? Of course not. It's not complicated to understand the story of Star Trek '09. Either you get it, or don't or refuse to play because this team had the nerve to make a new Trek film without your permission and decide unilaterally that everything is wrong. Apparently most of the people arguing the loudest and the most obstinately fall into the latter category. Stop hyperventilating, wipe the froth from your mouth and go have a nice lie down. We get it. There's no need to beat everyone over the head with it. You're entitled to your opinion, and the rest of us are entitlted to ours. OK?? Good.
Have a nice day.
Eat SnackySmores™