The cost of another country leaving may indeed be higher, but the EU simply can't afford to punish the UK, even as a deterrent. They're all talk and no trousers on that front.

I'm not sure what you mean by "special benefits". Yes, the UK negotiated the rebate and certain special conditions of membership, but fundamentally it was a net contributor to the EU.

I think the current EU is toast either way, so the argument of what deal the UK negotiates with it may end up being moot.