The argument doesn't have anything to say about what should happen at the nation-state level.
Concerns about social cohesion and stability are mostly relevant at the nation-state level. This is so because at sub-state levels the exit option is generally available and is viable. At the state level, not so much.
In plain words, it's much easier to move out if your town loses cohesion and stability than if your country does.
you don't get to choose the diversity
You don't get to choose the diversity, but you can incentivise or disincentivise the differentiation with long-term consequences. For an example, look at what happened to, say, people who immigrated to the US in the first half of the XX century. They started with a lot of diversity but because the general trend was towards homogenisation, that diversity lessened considerably.
If it's worth saying, but not worth its own post, then it goes here.
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