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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.
You keep mentioning the EU can't afford it, but why do you think it can afford to let the EU explode?
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.
That is not relevant. Access to the market will have to be paid for, so they still need to be a net contributor. Maybe even more than they did so previously.