Yeah, we are pretty much running into a significant logic problem if there is the demand for Spock to go back and undo the destruction of Vulcan due to the number of lives lost. It pretty much sets up a problem of, "Why not go back and save all the lives?" or "We only go back and save lives when it is so many." Either one presents issues because it makes the characters having to play almost a god role of deciding who lives and who dies.
It's kind of like what happened with Superman with "Man of Steel." Superman has the power (we've seen it) to go back and time but only uses the power when impacts him personally. So, despite the significant levels of destruction in "Man of Steel" none of it impacted Superman personally so no time travel. Same with VOY in "Endgame" where Janeway decides to impact many hundreds of thousands if not millions of lives just because she had a personal loss. Apparently going back and preventing Voyager from going back in the first place and saving her first officer, or even going back and stopping the Borg didn't occur to her. Only certain lives matter.
It's quite a dangerous argument to make.
Yeah, can agree with all that. Not something the franchise has invoked that much in the TV shows and movies, although the Sciences rule book for the Star Trek Advenutures RPG did devote a bit of the time travel section to the murky questions, like someone being ordered to let their daughter be wiped from existence because she wasn't "supposed" to exist. The old LUG RPG also had a similar campaign story idea where a time travel accident flings an early victim of a plague into the future, forcing the players to have to decide if they can condemn this person to the horrible life they would've lead for the greater good (curing the plague) or to try and find a way to ensure the cure is found without forcing patient zero to her original destiny. Something that could be quite interesting to see play out on TV in the franchise; the ethics of time travel and how far can you go with crossing the line.
However, I was thinking in a more practical standpoint; how would Spock even be able to stop the accident even if he had a time ship, set up a warp core implosion, or rigged up a sling shot? I don't see how he can make it back to the prime universe while being stuck in the Kelvin timeline to prevent himself and Nero from being sucked into the red matter black hole, and I don't see how he'd be able to stop Nero from destroying the Kelvin by going back to Kelvin timeline 2233. Also pretty sure Starfleet would stop him from meddling if they got wind of it.
So, yeah, I agree that Spock should've fixed the timeline if he could, but I don't see how he had the means to do that.