The way i understand it, the greater balkans outside greece, including the region of what is today Romania (Dacia), became latinized through roman dominance and influence untill its collapse and the adoption of greek as a substitute lingua franca by the Byzantines. The slavs who came into the region then managed to gradually assimilate the populations of the region like the Thracians and Illyrians, with exceptions of peoples either geographically isolated such as mountanous regions (Albanians, Vlachs), or peoples whose culture/language held a priviledged position in Byzantine society (Greek). I can't understand how Romanian has survived to this day, without being assimilated by another "barbarian" language group. The language is isolated, and the regions of Wallachia and Moldavia include large swathes of land vulnerable for conquest and domination by migrating people groups (Bulgarians, Pechenegs, Mongols) similar to pannonia, which itself became Magyarized.