What would it take for the Roman inhabitants of Keszthely to preserve their culture and language until the modern day?
I am developing a con-lang based around the survival of the Romans in what is now modern day Hungary, the historic Pannonian Romance language. I initially stumbled across
this older post, while helpful, was a bit too difficult to imagine and outside of the bounds I set. Unlike Planet of Hats, I am not necessarily interested in preserving the province of Pannonia in perpetuity. Rather, my project rests on keeping
some element of Keszthely culture alive until the modern era, much like how Dalmatian and Istriot survived, even if it is a small population of a couple thousand people.
Since the lifeline of the Pannonian Romans was connected to the Avars, my belief, like cmakk's in the other thread, is to save the Avars. Now, Wikipedia and Cmakk seem to suggest that it was the Frankish invasion of Pannonia that finally did the inhabitants of Castellum in. To be frank (no pun intented), I am quite skeptical of this. The changes were mostly political in nature; the Avars as an ethnic group did not disappear because of it. Instead I believe the huge influx of Hungarians arriving in the 10th century assimilated the Slavs and likewise the remaining Roman inhabitants of Keszthely.
My current idea is that the Hungarians were not victorious at the Battle of Pressberg, but suffered a crushing defeat instead. After several additional battles, the Hungarians eventually were resigned to east of the Tisa river. This POD has two outcomes: a Slavic speaking state arises west of the Tisa river and ecompasses much of the territory that used to be Roman Pannonia, and more importantly the Roman inhabitants of Keszthely (or *Kostel in this timeline), survive much longer, perhaps even to the modern day.
So my question is twofold: is this a reasonable justification for keeping Keszthely Culture alive? Certainly its not the whole story, I still have another thousand years to work out, but its a start. Secondly, what could cause a Hungarian defeat at the Battle of Pressberg? A greater Carolignian Empire? Thoughts are appreciated.