The only counterpoints I can think of in terms of old world (i.e., excluding the Americas since those are predominantly multi-ethnic, immigrant-based societies) successful truly multiethnic states are Switzerland and Belgium. Maybe India as well, though they seem to be sliding into chaos as well these days. There are probably some in Africa as well, though I have to admit I'm not particularly well versed in demographic trends there.

Nonetheless, none of the successful versions are anything close to a theoretical one state solution