King Daniel wrote:
Ah, but the rules sometimes DO change in-universe and what happened before is swept under the rug. Look at warp speeds - Voyager slowed warp speed down to make crossing the galaxy a 75-year journey despite lots of evidence that such journeys would take a month or so.
Warp speeds have always been ambiguous in Star Trek, even between episodes in the same series. Warp speed has always been the speed of plot, which is faster than light, but not faster than the arrival of the 3rd act.
Also, Orci and company have gone to great lengths to insist that this is all in the same universe, that TNG is still "there" in an alternate timeline. If so, major narrative devices which enable time travel are still there.
(i.e., devices which underwrote the possibility of time travel which were needed for a film or episode). That is, Star Trek IV still exists in Spock's memory along with the TOS episodes involving sling-shotting around the Sun. Spock, like everyone else in Trek, is fuzzy on details like how fast warp speed is, but NOT on the fact that one can use a star to travel back in time within the same timeline.
King Daniel wrote:
If the many in-universe reasons posted so far haven't scratched your itch for a plausible answer (and I think you're REALLY underselling how dangerous Nero was and how easily he could have succeeded) ...
If Nero was so unbeatable, he wouldn't have been beaten by nu-Kirk and nu-Spock.
Spock has faced much more intimidating opponents and never backed down before.
What matters is Spock has a solid chance to save Vulcan.
He knows Nero can be beaten. He knows how to sneak on to his ship. Hell, he's got the recipe for transwarp beaming, so he can beam aboard from a space-like distance while the Narada is at warp. It's nice big ship, but she has a small crew. nu-Kirk and nu-Spock were an effective boarding party. Take a hundred or so "emotionally compromised" Vulcans with you or beam aboard some H-bomb level weapons and detonate them, and the problem is solved. Vulcan lives.