Unlikely. Who do you think inhabited Bulgaria before the Slavs and the Turkic Bulgars invaded? Greek or Latin speaking Roman citizens. The Roman citizens likely outnumbered the Slavs and the Turkic Bulgars, and yet Slavic language and culture still prevailed. Those Roman citizens who didn’t assimilate were either resettled along the coast, or resettled north of the Danube (thus creating Wallachia). The same thing probably would have kept going on if Constantinople fell to the Bulgarians, I don’t think a Hellenized “conquest dynasty” scenario would’v happened.