OR, Spock Prime steps on a temporal butterfly, gets himself and his team killed. Bomb disarmed. Nero and his crew are more ready now and aren't stopped by the younger Kirk and Spock, and Vulcan is joined by the 130 other Federation planets.
And no, I'm afraid viable methods of time travel ARE as disposable as warp speed time/distance ratios in Trek's world. Just look at things like weapons - why weren't Genesis missiles shot at invading Borg ships in Next Generation, making cute little cuboid planets? Why didn't the Enterprise-E use the godmode armour and weapons Voyager brought home in "Endgame" to pulp the Scimitar in ten seconds? What ever happened to the Kelvan's warp drive upgrades given the Enterprise in "By Any Other Name"? Or the Enterprise-D's 300% shield upgrade in "The Nth Degree"? Or the ability for Klingon ships that could fire while cloaked? Or the wide beam settings on phasers, which could have ended "The Seige of AR-558" in two minutes?
Why didn't Spock use the Guardian of Forever (potential Harlan Ellison lawsuits aside)? Why didn't he go at warp 10 away from the supernova danger zone and slingshot back in time a single day and try again in 2387? Why didn't the TOS crew use their time travel abilities to undo the massive destruction wrought by the doomsday machine, Nomad, the the parasites on Deneva, the planet-eating cloud in the animated series? Etc.
Wow I just realized I know far too much about Trek.