Human change can be subject to the law of large numbers, but nothing necessitates any particular change being towards progress.
>Skilled polish engineers don't want to be the only polish person in the entire country. They want food, culture, community that reminds them of home. Even as they assimilate. That's why the American melting pot works well. It encourages enclaves that touch one another.
The American melting pot works well (or worked well) because it was a nation made up from a blank canvas with no prior historically established dominant ethnicity or culture the kind other nations have had going for millenia.
And even at that was built on first disenfranchizing (to put it midly) the natives.