To expand on a point I was making in the O.P.:
bluegenes in O.P. writes:
Two human population groups living in the same place will inevitably mix over time unless they practice cultural separatism
Let's look closely at the history of the Jews in Germany. They've been in the area for around 1700 years. This is before the time that a group we know as the Saxons left from the area to come to England and, amongst other things, give us the Germanic base of the language we all use here. I'm pointing that out to put things in perspective.
In England Saxons, Celts, Danes and later arrivals, the Normans, all mixed with each other to make one tribe, the modern English. As soon as important cultural differences (like who was Christian and who was pagan) were eliminated, they mixed quickly.
Christians and Jews back in Germany retained their separate identities through all this time because they were kept apart by their religions.
The history is interesting. Have a read here, and note the zealous religious slaughter of Jews that went on at the time of the crusades. (The same thing happened in England, and Judaism was banned until the 17th century).
History of the Jews in Germany - Wikipedia
Now fast forward to modern times. Because religion is far less important to both Christians and Jews, the existence of Judaism outside Israel is under threat. 52% of American Jews marry non-Jews, and surveys show that only a quarter of the children produced from these relationships are brought up in the religion.
This means that Judaism in the U.S. is losing about 3/8 of its adherents every generation. As you can imagine, this causes religious Jews great concern. Three or four generations of this, and the Jewish religion in America will be a bit like the Amish; a tiny group of Orthodox believers who marry amongst themselves.
This same mixing process could have happened at any time in the history of Germany had the importance of religion died down.
I use that example to illustrate how it was religion that kept two population groups separate in the area that we now call Germany for an incredible 1700 years. As I said in the O.P., population groups living in the same area/country will mix over time unless they practise cultural separatism of some kind. Religion is only one kind of cultural separatism, but it's probably the strongest, most profound and most effective known to history.