Uh, since when did any "western" state have "unmanaged immigration" going on? I mean, even the illegal kind is sort-of managed by the police and whatnot on account of being, you know, illegal.
After something like the 1700s anyway.

Also, the monoculture thing was sort of tried, round late 1800s to early-mid 1900s or so. It failed quite spectacularly on account of totalt dissociation from factual reality, but not without doing kind of a lot of kind of seriously ugly damage. That kind of didn't leave that many alternatives for civilised people.