> How do you know if Apple will be able to sustain an app store with that number?

I suspect part of the problem is that Apple could be creative with its finances and make sure that just about any cut that isn't 30% means they can't sustain an app store. Apple earns an insane amount of money, they could get by with 0% commission just fine, but it's not in their interests to ever admit that.

Sure, 12% is made up, but the result of just about any negotiation is a made up number that sits somewhere between what one party wants and what the other party demands. It's just the nature of it.