> This is the other part: if associate membership is a thing then won't half of the EU start thinking that's their future?
Presumably by "half of the EU" you mean (sometimes) euro-sceptic states like Hungary or Poland? Well, these are not in EU because some illuminati/lizard people tricked them into joining and now won't allow them to leave, like some of their politicians would like you to believe, but because they enjoy billions of subsidies per year.
There is a strong case that much of the eastern bloc were tricked into joining, by the UK, in order to weaken the Franco-German efforts for ever closer union.
This was even a joke in the UK at the time. https://www.youtube.com/watch?v=ZVYqB0uTKlE
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I would like to make a slight correction: the hungarian public opinion was never, ever euro-sceptic. This was one of the major reason the previous government was voted out, because they didn't reflect the will of the people. Opinion polls regularly poll 70-80% of EU approval despite being decade-long propaganda trying to change peoples' minds, making Hungary de facto one of the most pro-EU countries in EU.