But the fragmentation would create new priorities where ideological alignment would form. There is benefit to numbers in union, but the sovereign cost should be routinely weighed and measured.

Sure, but the point is, if you can just leave - there is no point in allowing yourself to be ruled by people with whom you disagree. And there is not really a logical stopping point there. If it's in terms of costs and benefits, it's unquestionably better for the South to have been part of the US than to be its own independent nation.