It seems pretty clear what should happen here: Apple should be able to require you to accept their payment system, but not to require you to charge uniform pricing across payment systems, and not be able to charge you anything for sales outside of their payment system.

Now you can continue to use Apple's payment system all you like, but if Apple continues to charge 30% when e.g. Stripe charges ~3%, you're going to pay the difference for the privilege.

And with any luck that would encourage Apple to match Stripe's fees, but either way, now the choice is yours instead of the extra fees being hidden and mandatory.