At the very least, they would need to decouple the payment processor in a way that still provides the user all of the following;

  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

And probably many more technical measures. Otherwise, you are directly damaging a key value proposition provided by Apple and demanded by their users.

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.