There is a vein of thinking that “diversity causes division” in the United States and this undermines our political willingness to create effective universal safety net programs.

High trust vs low trust societies tend toward egalitarian vs clannish communities respectively.

it means society can only thrive pre-integration or post-genocide/ethnic cleansing.

For example, Northern Ireland cannot exist in the successful European model until the Catholics are gone, or all the Protestants are gone.

Which is a strange thing to claim because e.g. in Germany, Protestants and Catholics "coexist in the successful European model", whatever that means. Claims of "can only thrive" are trivially refuted by counterexamples.