Europeans are going to have make a choice between immigration and socialism. It was socialism that reduced their birth rates to below replacement levels and they were told that skin colour and culture did not matter. The immigrants would adapt, they said.
Socialism didn’t “reduce the birth rates to below replacement levels” - worldwide we see birth rates collapse across all societies when the society gets better educated and richer. Socialism does not seem a factor in that particular issue.
As to skin color and culture did not matter — in general it does not matter if the immigrants are any religion except Islam and are a culture that is accommodating —> case in point, there are Christian Middle Easterners in Europe who have assimilated to each country pretty well. And there are Christian brown skinned people from South asia who assimilate pretty well.
OR even brown skinned Hindus, Sikhs, Buddhists etc. - they may remain in their religion or not, but they assimilate well enough to not threaten the hosts.
Multi-culturalism worked in the USA between English+German+Irish+Poles+Italians etc. — even Chinese immigrants to the USA merged pretty well. Multi-culturalism fails when Iz is brought into the mix...