Populations rise and fall naturally. Europe lost half its population in the middle ages. Importation is NOT the answer.
Europe also lost a lot of people in WW2
The welfare state burden was unheard of in WW1 or 2 or medieval times.
For the welfare state to stay constant it needs an never ending 'next generation' of greater proportion than the last to keep the gravy train rolling.
The combination of a death spiral rates and Countries refusing to curtail spending has left them with no choice but to import baddies.
They are in a no win situation.