Well then the rule could be: All apps must offer the possibility to use Apple Pay for payments.

Or Apple could do one of those virtual credit card things. Or do they already? So you could use an Apple virtual credit card with any processor.

But then the 30% extra fee would be visible to the user. Apple's whole schtick is making things way more expensive and hiding the competition pricing from the user. If "$10 with Stripe" was placed side by side with "$13 with Apple Pay", Apple would immediately lose all of their payment customers and about $2,000,000,000 in revenue, which is a lot. They can only prevent this by preventing "$10 with Stripe" from being there at all. (And if they did it with virtual credit cards, they'd have to tell users there's a 30% fee on each transaction)

That’s an unsupportable claim.

If customers want to pay more for Apple to facilitate, there isn’t an incentive to not keep apps in the App Store too. If a company has other reasons not to be in Apple’s store, why should they be required to?

this is about allowing business to offer other choices, other store rules still would apply. for example if apple would be to require it's option as always present together with the rest in the app, then noone could say anything against it. it's the lack of choice that is the problem not requirements per se.