What would one or both of these stores need to do, to qualify as 'anti-competitive' to you?
That's a good first step towards competition that would allow Apple to keep the AppStore exclusivity -- for now.
1) No need to re-enter billing information across apps
2) A centralized UI for the user to manage all their
subscriptions, enforce consistent billing policies,
request refunds, etc.
3) Enforce parental controls and family approval
policies which may be configured on the account
4) Prevent any surprise billings or unauthorized charges
5) Ability for Apple to force a refund even if the app dev won't
In the end, the payment processor is a red herring. Apple isn't charging 30% for the payment processing -- they probably pay less to run the charge than Stripe does. For example, for $0.99 charges, the app developer pays Apple less for their entire fee than Stripe would charge them just to run charge.
The 30% fee is Apple's share for creating the entire ecosystem. Everything from the programming language, the platform, the API's, the security framework / secure enclave, the app review, the distribution model, the billing system, the cloud services for software delivery/push notifications/updates/shared storage, all the way up to Apple's customer support lines. Actually running the payment gateway is perhaps the very least of it.
Not in the context of introducing competition.
IMO, as long as there are viable competitors to Apple, and as long as Apple is not colluding with its competitors, Apple should be able to implement any lawful policy on its own device that it chooses.
Apple can't violate its existing contracts with its customers or developers (e.g. threatening to kick off all Unreal Engine apps, e.g. lock customer data unless a ransom is paid). Apple can't use App Store policy to commit crimes, like extortion (e.g. offering to buy an app developer, and saying they will ban them from the store or increase their commissions to 50% if they don't accept the buyout).
As long as their policies have a justifiable and legal business purpose (whether you personally agree with the policy is irrelevant) that's applied consistently (within human limits of consistency) they should be free to make their own product however they want, and charge what they want for it.