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.
It's culture and values come from the dominant group. It didn't really mix equally between groups. If the dominant groups will shrink and another arises the values and the culture will change.
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.