Monroe wrote:"threaten the Galaxy" could mean simply the peace and stability of the Galaxy by the sudden power vacuum.
I think perhaps the Alpha Quadrant would be more peaceful without the Romulans, although Nemesis seemed to indicate a general trend towards pacification. I think though, that without Romulus we would see the ascendancy of the Klingons and Cardassians even more so.
Then the effects should have shown a gamma ray burst, not a supernova. A supernova expands outwards in all directions; a gamma burst would have been in single direction, and the odds are hugeley against being baked by a gamma ray burst anytime soon.Or it could mean the star would have a Gama Ray burst which would cause huge amounts of the Romulan Empire to suffer which would threaten the Galaxy for the same reasons.
You can't win guys, they goofed with the whole supernova crap, end of story.
But, like I said, still a great movie.