Paris is right. It's a fallacy to equate the planet Romulus with the Romulan people. Nero may have claimed he was "the last of the Romulan Empire," but that was surely the hyperbole of a grief-deranged man. I mean, it's a star empire. An interstellar civilization. Of course they didn't all live on the planet Romulus itself. Just because Romulus is gone, that doesn't mean there's no more Romulan civilization. Heck, we even have canonical precedent for species that have lost their homeworlds but still exist, from the Platonians and Fabrini to the Suliban and Xindi.