Wha? There are ways around this. Apple could just build sanctioned support for specific payment gateways (e.g Stripe/Adyen) and subject them to rules as a part of the app review process in the same way they are now. It doesn't need to be binary; it's just that they want to paint the picture as if it'll be a wild west of payments if they have to change that, even though they have the power to build safer alternatives.