Wow, muh Hungarian cuckoo-land fantasy in a single paragraph.
The burden of proof that the Romanized population moved anywhere is on you. Genetic testing has shown conclusively that migratory populations were always the minority in the territories they settled, including in the Carpathian basin. Even the Slavic migration, the most numerous migration of all in this area, was a trickle over centuries. We know that in the region it was first the Scythians that became Slavicised, then the free Dacians (the non-Romanized Dacians that lived outside the Carpathian arc created with Slavs a common culture called the Chernyakhov culture), then Romanians and Bulgarians. This took literally a thousand years.
Again, if we look at the graves, the Roman graves outnumber the migratory people graves by several orders of magnitude, and we have dug only a tiny fraction of the Roman cities in Romania. It's just no comparison. So it's no surprise that the genetic backbone of the population remained paleo-Balkan (Roman Dacians were Romanized paleo-Balkanites). Romanians plot closest to Paleolithic samples from Transylvania, out of all the people in the region. How come Romanians match the Paleolithic sample if they migrated to the place later? the migration theory makes no sense. In addition to the genetic proofs there the cultural arguments. All major river names in use by Romans in 100 AD were still in use in 1000 AD. What population preserved those names?
This is a typical Hungarian red herring. There are almost no contemporary sources talking about the area to have reached us. This doesn't mean there weren't any, it just means they were not preserved - this was the fate of most material written in the early middle ages. The dissolution of the Roman empire also decreased the likelihood of such reports to be written and survive to this day. The migratory population presented some interest because they were a military threat. Even so, descriptions of the material life of the migratory people are very scarce, brief and contradictory, that we still have trouble identifying them.
Most Roman cities in Romania were abandoned by 600 AD. This does not mean Romanized people left the area, because they are mentioned again when Bulgarians extend their domination over the Carpathians, around 800 AD. Again "absence of evidence (for 200 years) is not evidence of absence".
In the 4th century here were only a handful of Christianized Goths in the area and most were killed by Goths themselves. Goth were Christianized in significant numbers a few centuries later in the West. By that time Goths have left the Carpathian Basin.
Are you talking about Upper Danube or Lower Danube? Lower Danube is in Romania. There are many early churches (basilicas) in South of Romania, though not many have been found in Transylvania. Even so there are early churches in Transylvania that were built right next to Roman temples of Jupiter, or even on top of them, showing that it was the Romanized population that used them. Again you are confusing your ignorance with proofs of absence.
Take for instance this Roman temple from Transylvania that was converted to a church in the 6th or 7th century:
Roman inscriptions on the stones of the church:
the church itself (note that the tower was added much later, perhaps in the 12th century)
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