unlike the Roma and the Jews, the only two populations that moved into Europe in significant numbers before the second half of the 20th century
Except for the Arabs who moved into Spain until the Reconquista. Or the Turks who moved into Romania and Bulgaria until the end of the 19th century. Or the Golden Horde, centuries before.
the people now moving into Europe will somehow cease to be non-European in nature
And what makes them non-European? Their cultures? Or the fact that they are darker-skinned?
If the issue is their culture, an individual can be a part of two cultures. My family has been French for about a millenium without interruption. My wife is of African descent, and yet, she has more knowledge and has integrated into French culture, more than my moronic cousins, who are uncultured and proud of it.
If what bothers the morons embracing this theory is that they are darker-skinned, there is nothing to add.