Epic Games has filed a lawsuit [1] and published a Fortnite-themed parody of Apple's "1984" [2] to get some publicity for it.
[1] https://cdn2.unrealengine.com/apple-complaint-734589783.pdf
[2] https://www.youtube.com/watch?v=euiSHuaw6Q4
They will lose as the TOS clearly indicates the rules and Epic agreed to them. This is basic contract law. Apple has a massive team of experienced lawyers.
I'm guessing the crux of the argument will be that the ToS are illegal in this respect?
It is likely extremely difficult to win such a suit based on challenging the terms of an agreement that you agreed to repeatedly. Every single change to the TOS for a developer account requires acceptance. I doubt there is any part of this agreement that Apple did not write with this exact circumstance in mind. No matter what Epic challenges based on this argument in the end they will give in and look like fools. Maybe if they are lucky Apple will lower the % by a little. This is not cutting edge law here.
I'm not sure why you feel qualified to comment on the legal aspects on this case based on just a primitive understanding of offer and acceptance (which isn't the point of contention here). You can't have an enforceable contractual term that is illegal, even if both parties have agreed to it. The legal issue is whether Apple's ToS contravenes some aspect of competition law.