Upvote:12
I think the question comes from a misinterpretation of the expression "Romans abandoned". This was not related to any substantial movement of population. This only means that the Roman army (as an organized force) abandoned the defense and law enforcement in certain places. The population remained the same, part of it "Romanized", but not all. The army itself partially withdrew and partially dispersed, and in any case, orders from the Empire were not obeyed any more. The process of destruction of the empire was slow and gradual. The people who lived then, in most cases, did not notice that the empire "ceased to exist" at a particular moment.
EDIT. I was asked to add references. Of course what I wrote is my own (condensed) opinion. But it comes from a lot of reading. Tons of literature exists on the subject beginning from Gibbon's Decline and Fall. So I just list some of my recent favorite books:
B. Ward-Perkins, The Fall of Rome And the End of Civilization, Oxford UP 2006
Peter Heather, The Fall of the Roman Empire, Oxford UP, 2006.
A. Goldsworthy, The Fall of the West: The Slow Death of the Roman Superpower.
Several authors emphasize that most residents of the Empire were simply not aware of its "fall", that is of the displacement in Ravenna of Romulus Augustulus by Odoacer. They did not know and did not care much.
And let me add two excellent works of fiction:
Iain Pears, Scipio's Dream.
Pascal Quignard, On Wooden Tables: Apronenia Avitia (transl. from French).
And here is a contemporary source: A poem of Rutilius Namatianus, he was a Gaul employed in Rome during the "fall of the empire", and he describes his travel back to Gaul.
Unlike Hollywood movies, good literature can be very helpful in such questions: A serious writer who studied the period tries to imagine how it really was to live at that time. History books do not always give you this feel. But a good writer can do this after having digested many history books!