Historically, humanity evolved faster when it was interacting. So groups can try to isolate themselves but on the long run that will make them lag behind.

US benefited a lot from lots of smart people going there (even more during WWII). If people start believing (correctly or incorrectly) that they would be better somewhere else, it will not benefit them.

Thing is, if there's too many of "them", they will eventually come for "us" with torches and pitchforks. You can victimize a large part of the population like that, but not a supermajority of it.