I see the destruction of Romulus being Hobus as being akin in its impact to Romulan civilization to--say--the destruction of the Eastern Seaboard in the United States, or the northeast of China, or of the Sao Paulo-Rio de Janeiro corridor in Brazil, or of the Moscow-St. Petersburg corridor in Russia. Most of the country is intact (mainly because it's such a big country), most of the population and economic base even, but the core of the old state including its main economic, political, and cultural centers have been destroyed. The polity that's left lacks its central institutions: the surviving components (military units, planets, other political entities) are left to cope as best they can and to try to reassemble some central government.
But these are not imperial analogies, either. Rome and, later, Ravenna and Constantinople's various sackings and near-destructions did not stop the empire from working.
I'm not disagreeing with you that Romulan culture and civilization would survive the destruction of the Romulan homesystem. I've said here on the past that the odds are the Romulans, who've been aggressively expanding their presence across space ever since their generation ships arrived in Eisn space, are present in very large numbers in settlements far from Romulus.
The scale of the disruption to the Romulan polity is much greater than that associated with the sackings of Rome and Ravenna and Constantinople. Despite heavy human and material damage, those cities remained functional enough, as did their various hinterlands, and could be restored. What happened with Hobus, which destroyed the Romulan homesystem entirely and who knows what other worlds, would be more comparable to Mongol hordes coming to the Italian peninsula or the Aegean basin and successfully destroying everyone and everything they found. That scale of destruction is something beyond anything that happened to the Roman Empire in any of its phases.
I think that the Romulans (I just realised that terrible corrolation of analogy and fictional race) - if just losing Romulus and its local system(s) - would survive. Different - there might be a St Augustine bemoaning the loss of home - but still carrying on. It would be poor world-building to see an empire fold because of the loss of a capital. If the Romulan Empire is indeed an empire, in more than name only.
Romulan civilization would survive, but it's not obvious that the Romulan
Star Empire would survive the destruction of the Romulan homesystem, with its population and wealth and political centrality. (Other Romulan worlds may well also be destroyed by Hobus. If these worlds were near Hobus and Romulus, it's possible that they may have been relatively important first-generation colonies, worlds like Romii. In this case, the damage would be still greater.)
