You have to read Mark Steyn’s book, America Alone, to get the data infused with some satiric zingers.
Net message is that Europe is absolutely incapable of returning to its native stock roots because of their birth rate. Whole society of nations committing voluntary suicide because of the pill and selfishness.
Obviously it would take some management, but no one has explained to me why a lower birth rate leading to a smaller population is a bad thing for a country. No reason it can’t be managed. Especially for advanced economies as in Europe. Though business might have to rely on more expensive labor. And perhaps some lowering of standard of living for older citizens. I’d suggest that’s the reason, I think it is relative to US immigration, but in Europe the immigrants don’t work, they collect welfare.