I agree to some extent, but how many US states would take the option to leave the union if they could? They benefit from being part of the US so immensely that

I agree to some extent, but how many US states would take the option to leave the union if they could? They benefit from being part of the US so immensely that it is hard to imagine conditions where they would actually leave. If the EU integrates more, and is sufficiently uncooperative with quitters, leaving really would be a last-resort option. I would argue that the UK is a special case, since its ties with the EU were lighter than basically all other members, but even then Brexit will quite possibly hurt it. From Germany to Estonia, member states have a strong incentive to remain.