Study history and scenarios where countries had heavy immigration and no assimilation and tried to “co-exist” together and you will find your answer. They don’t assimilate; one culture conquers the other and wipes it out. The bible talks bout it quite a bit but if you’re athiest and think the bible is just fairy tales go look up the history; it’s out there and says the same thing.
The Balkans are a prime example of your point.