alt Hacker NewsDidn't downvote but I'll bite: Haven't states largely always been ethno-states, and isn't "ethno-state bad" a relatively modern and potentially as-yet unproven argument in the big scheme of things?
It's clearly bad to specifically re-engineer the ethnic composition of an existing state to create one, particularly violently, but I don't know that it necessarily follows that a naturally occurring ethno-state is conceptually abhorrent as you seem to be suggesting. Japan is very much such a state - is Japan bad for existing?
What is modern are states like we have now. Passports didn't exist before WW1 for example. The idea that people can't freely move across land and imaginary lines we call borders is a modern idea.
You have it backwards.