>> Also, game theoretically it's of course more important to not make leaving (or threatening to leave) the union seem too lucrative.

Because you simply wouldn't be offered the option of just deescalating your full membership to an associate membership. If you want to leave, you leave fully, and then in time we can discuss what happens next. Also, obviously an associate membership shouldn't feel more desirable than full integration if you're eligible to both. Indeed from the UK's perspective it would be exactly the opposite to what they wanted: free labor and movement but no representation.