Both interpretations are possible, but of course we're not talking about some universe that actually exists and we're just learning about in pieces; we're talking about something that's entirely made up. So whether there are few Romulans left or many is solely a function of what the writers want to establish about it, and the writers can select the facts to fit the version they want. So we can't really argue that either scenario is certain. It could go either way just as easily.
But since we're unlikely to see any more canonical depictions of the Prime universe post-2387, that leaves it up to the tie-ins to determine how things go. And my point is that there's nothing to prevent
Pocket or IDW from establishing that the Romulan people still endure after Hobus. Indeed, as Markonian
just pointed out, Star Trek Online
has already done so -- in their continuity, the supernova threw the empire into turmoil and triggered civil war and political strife for quite some time thereafter, but the RSE still exists in 2409 as a major galactic power and there are still plenty of Romulans around. I see no reason why the novels wouldn't take a similar route, especially since that would be in keeping with how the novels have already depicted the RSE to date. The scenario of a post-Hobus reality where the Romulan culture is extinct just doesn't seem like something we're ever likely to see in any professionally created work.