Saying that saving six billion Vulcans is reason to go back and undo everything is BS. Why? In the Prime future at least as many Romulans and Remans are dead. If you "fix" the timeline they all die again. In the alternate reality, with foreknowledge of the Hobus supernova, all those deaths can still be averted.
Billions die in both universes - probably more in Prime because of the destruction of Remus too. Can you imagine Vulcans wanting their world saved at the expense of two others? Even those of their enemies?
Spock going back, saving the Kelvin, saving Vulcan AND sending a secret message to the Federation and the Romulans to beware of the Hobo star is not helping?
Spock once went back in time to get some whales to save Earth. So he wouldn't have a problem with warning Romulans about the supernova, while he's already in the past.