Romulus was originally a colony
...Or was it? Vulcans supposedly do not originate from planet Vulcan (TOS "Return to Tomorrow"), and we don't know exactly how this is evident. Were the origins of life on Vulcan the result of transplanting from some other world? Or, since the TOS episode mentions a transplanting civilization that lived mere hundreds of thousands of years ago, rather than millions, were the already evolved humanoids the thing that was transplanted? Nothing suggests that Romulus would have been the original homeworld, but it's a possibility among others.
Certainly Romulus could be an ancient colony, at least two thousand and possibly three thousand years old by the time of the eruption of hostilities with Earth. Much would depend on whether other worlds within the Romulan sphere of influence would have been allowed to grow to strength similar to this capital planet; quite possibly, Romulus had true "homeworld" status in every respect, while all other worlds were small "colonies".
All this said, it's still a matter of "civilian" losses first and foremost: lots of skilled people and leaders dead, lots of industries gone, lots of despair that isn't moderated by the fear inherent in the "military" structure of the Star Empire. But the military would be drastically weakened by these losses, too. I doubt the colonies would have been allowed to have infrastructures or skill sets allowing for the maintaining of a starfleet, so the ships would be on their own - a tolerable situation for a decade, considering the independence of Star Trek ships, but intolerable in longer term.
Let's also not forget that the Trek empires do not exist in a power vacuum. The Romulan Star Empire might collapse at the first sign of weakness because it's under constant threat from its neighbors. It's a bit of a miracle that it doesn't lose planets every time there's a political scandal or an assassination at the centers of power! (Or perhaps it does - we don't really know what happens in those constant wars between Romulans and Klingons...)