If DoS was indeed the very last chapter of the Dracula-Belmont war, it's fitting they would die with their biggest enemy. Apparently there's always going to be a Dark Lord, but that's not technically what Leon signed them up for.
If they're going to wrap up the continuity some day, I think they need to clarify what happens to the Belmonts. But cloning / genetic engineering to create Belmont successors isn't that far fetched. I mean, obviously the world was aware of Belmont blood, which made them superior vampire hunters.
But that's kind of hazy. I mean, Leon was never implied to be in any way supernatural. Did his descendants gain more power because of the ritual Rinaldo performed or was it all just genes, training, Vampire Killer and all other mystic weapons? Before LoI, at least, they implied their power was due to "warlord chromosomes".