But are the Romulan colonies puny, or are the population and resources more evenly distributed? I don't think there's an answer in canon, but I picture it as reasonably well distributed. The Romulans were powerful enough to turn the tide in the Dominion War and too powerful for the Federation and the Klingon Empire to deliberately pick a fight with or conquer. How would they maintain a large empire without a great many powerful/resource rich planets? I don't see all of that power coming from one planet with a few puny colonies.
Also, taking out Romulus might lead to the government relocating - and being attacked en route by a dozen different factions wanting to be the next government.
Would this be much more dangerous than usual? They probably guard against assassination on a daily basis, being the paranoid people they are. And do we know that there are a dozen powerful opposing factions just waiting for an opportunity to pounce? I'd think the Romulans would pull together in the face of a crisis. They're xenophobes - their first priority would be maintaining their borders and not looking like a target to their neighbors.
At the end of the week, there would be sixteen new capital planets and fifty-eight separate warfleets of changing allegiances
If this were the Klingon Empire, sure. A hundred different clans would battle it out. But the Romulans always seemed more united than that. They had their problems, but they didn't give the impression that the empire was on the verge of collapse. Quite the opposite, actually.
But if you're starting out with the "puny colonies" viewpoint, we're not going to agree. Your argument does make sense if the colonies are small.