Part of the Nazi ideology was the idea that Slavs were ethnically inferior and therefore the Nazis felt justified in trying to slaughter them and take their land. In just the same way that colonists had taken Africa, America and Australia. It's just fortunate for them that the Russians had tanks whereas the American Indians or Belgian Congolese etc didn't.
And of course the people in the middle east have been so peaceful in the same time period.
https://en.wikipedia.org/wiki/Californication_(word)
Given the observed rates of demographic change, it seems very foolish to expect the "gold pocketwatch and pension" of 1950s to continue in perpetuity in the UK or Europe in general. For example when Europe goes full on Islamic how does Sharia law work WRT legacy 1950s era western pensions.
You can't wipe out a population, replace with another demographic, and pretend magic dirt means nothing else will change. Look at the lifestyle in Jewish Ghettos in Poland and Germany before and after the Holocaust for an example. Or look at Manhattan island two hundred years before the euro colonists vs two hundred years after the euro colonists. Thats all I'm saying. Regardless of right or wrong we're waving goodbye to what was Europe and it seems extremely foolish to assume legacy euro retirement programs will exist in the future due to magic dirt.
The "why" and who's behind it is I'm sure a fascinating off topic discussion but the UK government's own rather linear chart of % non-UK school children vs year the data was gathered has certain implications, if the linear trend continues.