Without immigration Europe would have already experienced a demographic collapse.
A society needs on average a fertility rate of 2.1 children born per female.
My country (Austria) has a fertility rate of 1.44, Germany even has a fertility rate of 1.38. Without immigration the fertility rate would be at 0.9 - 1.1.
This would be equal to or lower than the worst fertility rates the Russians experienced after the fall of the Soviet Union (1.1), when even life expectancy fell by 10 years on average.
Today with much better socioeconomic conditions Russias fertility rate has risen again to 1.8. That should give a clear indication of how important socioeconomic conditions are for reproduction.