This is the other part: if associate membership is a thing then won't half of the EU start thinking that's their future?

Maybe, and that might actually be okay from the EC’s pov. There’s a fairly old proposal for a ‘two speed EU’; a core that accelerates towards near-federalism with a more eurosceptic periphery.

There is always the debate around "Europe of two speeds" - where there is a deeply integrated core of nations, maybe even going towards "United States of Europe" and the others tightly associated, but not fully integrated.

Of course the set of countries and level of integration depends on whom you ask

>> 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.