I never knew that the entire heart of the Roman Empire disappeared in a matter of seconds. I'd swear that Rome still exists. You learn something new every day.
Of course any analogy we come up with isn't going to be exact.
But Rome was not in the Roman Empire for the last 1000 years or so of it's existence, it was 'sacked' which means politically it was destroyed in a very short period of time, maybe not seconds, but allowing for poorer communications, it was politically the equivalent.
The only conceivable way that Romulans are gone after the destruction of Romulus is if they all
lived on Romulus, which as we have already established, is unlikely in the extreme, and, in the novel continuity, which is likely to be the only one where the question is ever addressed, already stated not to be the case.