There are a number of examples of societies that quickly became multi-cultural.2. That different people, not just racial differences, but, different people in general cannot form cohesive communities. And, once more, by cohesive, I mean societies were individual freedom is optimized and state inference in minimized.
1. Fiji, where the population is majority Indian due to British labour importation. They have had about seven civil wars and are currently in a quasi-police state.
2. Bosnia, an artificial country where 90% of people vote for ethnic-interest parties
3. Guyana, where the country is in a political ethnic deadlock between those of Indian and Afro origin. Both are Marxists
4. Suriname, the same as Guyana, but lesser division
5. Malaysia and Indonesia, both have laws discriminating against the Chinese minority. The opposition far-right party in Malaysia wants affirmative action for the Malay majority over the more successful Chinese. About 1 million Chinese were killed in Indonesia in the 60s, for the crime of being Chinese. I have heard Jordan Peterson say that 3% of the Indonesian population are Chinese, yet they control about 75% of the wealth. Not checked that out.
Those are ones with large minorities. The lot of small minorities in many non-Western countries is much worse.
Much of the left are living in a fantasy world going down their route. They don't get their self-deprecatory values back at themselves now and they won't later.
The best example they have is Singapore, where Malay-Chinese tension is low, but they largely self-segregate. But the left hate Singapore.