I always interpreted Spock's description of the Romulan supernova being a threat to the galaxy as a figurative description, not meant to be taken literally, of the political aftereffects that the destruction of Romulus and the fall of the empire might cause.
We know the Romulan refugees caused splits in the Federation that led to a secession crisis and resulted in Starfleet pulling back.
What we don't know is whether the fall of the Romulan Empire caused a civil war within the Romulan Empire for control or border wars. I could see the Klingons taking advantage of the situation to annex Romulan systems, with basically the situation from TNG's "All Good Things..." happening in a way.
So that's how I interpret Spock's description to Kelvin Universe Kirk, that he was trying to stop an event that could have galaxy altering consequences that would threaten billions of people far beyond Romulus.