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?