As such, it is far from obvious whether certain European countries will have economies and militaries strong enough to remain reliable allies
Translation : the US only want to ally themselves to countries which can be useful. A country may share the US’ values or interests, if it is not strong enough, the US do not care. The US do not want allies. They want auxiliaries.
in 20 years it's going to be unrecognizable
Hardly. And this declaration ignores the fact that Europe has been changing throughout its whole history. The Romans had an empire covering Northern Africa, with inhabitants moving around. We had people coming from the central plains of Asia, at one point. Spain was under muslim (and Arabic) for a good long while. Bulgaria was under Turkish rule up until the end of the 19th century. Bloodlines in Europe are intertwined in unpredictable ways. The only ones who actually care about them are the morons with so few accomplishments to their name that the only thing they can take pride is who their ancestors fucked, as if they had any role in that.
My mother in law came straight from Africa to Europe. She worked her whole adult life, raised three children who all have good jobs and contribute to society. My wife has more of a right to be in Europe than all the parasites who want all their needs and wants taken care of just because their parents were the right skin colour.