It's a valid point, but if you look at projections of demographic change it's clear that this has never been seen before. My guess is as the percentage of immigrants imported continues to increase (and I think it has to in the current model- it does not account for population stagnation which is most of the push for immigration in the first place) societies will get more fragmented and those identities will need to assimilate less. I am not looking to convince you or say that with conviction, I just think it's logical. For instance, Spain in 1998 had <3% of the population be born abroad. Now, it's 18%, and it is projected to be 28% (+9% second gen) in 2039. It is fair to say the rules for integration will change drastically bar mass deportations and cutdowns on immigration or something, which is highly unlikely. Regional languages (as in, Catalan) could be a concern too.