The USA is pretty dang diverse, and definitely mixed. That mixing is a core part of it’s culture and history, actually. And now, the country that mixed (USA) is stronger than the Old World that didn’t (Europe).

Which is to say: There is no inherent psychological tribalism that makes having a diverse country impossible or ruinous; rather, this tribalism is manufactured and spread by hatemongers. The New World is a very good case study, here.