Rome spent serious time, blood, and treasure to conquer and hold Dacia down over a couple of centuries; Aurelian (one of my favorite emperors) gave it up, resettling Romans from Dacia into territories s of the Danube. If not for Aurelian, there would have been no successors, Rome would have been finished a couple hundred years early, there would have been no Byzantine empire as such, and European, n African, and Near East history (at least) would have been quite different.
There's no controversy that the Roman Empire made it fairly simple for ethnic groups to relocate within and outside the borders (people from India lived in Rome, Romans joined Greeks already living in India, etc), there's been some DNA studies of human remains of that era, just not from Italy. There are very few known human remains from Roman Italy, and most of them haven't been studied at all (DNA studies don't always work, there's not enough material to study).
The theory I’ve always been led to believe (from Romanians nonetheless) is that when Aurelian vacated Dacia, he only took the administrative people with him. The actual Romans citizens still living there chose to stay back and continue living in the vacated province for centuries. And they continued to have commerce with Rome via Moesia and Pannonia.
It’s an interesting theory because after Aurelian vacated, Dacia disappears from history for about a thousand years. In a way, it went through a similar, yet even longer transition from Roman Province, Dark Age Period, to Middle Age Kingdom as England did.
I lived in Romania for several months and there is no denying that modern Romanians are of Latin descent, just based on looks alone. The only Romance language I speak is Castillian, and after a week or so I could probably understand half of what was said to me.