1/ For the US, it’s too late already. The idea the North America will be a bastion of pure white, west European (being liberal, most preferred British) Christian culture is gone. It was gone even before they let us in.
2. The Europeans are dying out anyways. They prefer to enjoy life and don’t have enough faith in the future to want to have children. That’s why the Germans are anxious to recruit educated refugees from the third world – they need someone to work. Same thing is happening to the secular ruling class in America, but in America we have lots of religious fanatics willing to produce anew generation (not to mention millions of people in adjacent countries with similar cultures to our own).