I don't know why cultural appropriation gets a bad reputation. It's how cultures grow. In an ideal world, this would function like this: you see something cool or more effective that some other culture does, and then you do it too. It's this model that attempts at assimilation should follow. Encouraging new groups of people to build enclaves instead of absorbing them into the wider population is problematic.
Imagine a country is a giant amorphous jelly like ball. Say your country is a red ball. Right now, when blue people come in, they make tiny little blue balls inside your giant red ball. Instead, they should be absorbed into your red ball, so your red ball gets a little purple.
Oddly enough, colonization and imperialism occasionally did this right. Don't get me wrong, most of it was horrible for the people involved, but you've got to look at the good as well as the bad. If you're a human, your ancestors lost to a foreign culture at some point. I sure as hell wouldn't agree with how any of my ancestors lived. Hell, I disagree with living the way my parents live.
There's a curious effect in the United States. Millenials move to cities where people in their subculture are more common, and in a way, make themselves more homogeneous. Sure, these new subcultures are not necessarily connected to ethnicity or nationalism, but it's still a division that will probably threaten stability in the future.
