Earlier quoted context omitted.
While in principle I agree with you, if Epic signed an agreement that they owed Apple a percent based on initial terms, then that is out the window.
https://en.wikipedia.org/wiki/Unconscionability It's called an unconscionable contract. Epic's argument is that you cannot enforce unfair things even if you signed on it. Of course, the court is going to decide if it's really unfair, neither Epic nor Apple.
I would be interested to see a solid case decided on unconscionability where both parties are multi-billion dollar companies with substantial resources to spend on negotiating contracts.
If "I really want what you're selling, but it's kinda expensive" is the new standard for unconscionability, then we're all in big trouble.