I'm siding less and less with Apple on this.
Many payment providers support integration into systems/websites/apps (via an internet API), so Apple could easily support multiple payment providers through the App Store, but then the developer needs to be charged a fee for that ability up-front. It can't be free. And payment can't be handled entirely in an outside system, because them how will Apple know that a payment has been completed to release the app or in-app purchase to the consumer?
Unless done right, it will become very messy for users, more messy than many ad-supported apps currently are.
Apple is not wrong ... trust will degrade. The motivation can't be to bypass the 15-30% fee. Apple just needs to recuperate (at least part of) that revenue in another way directly from the developers. There's a cost for Apple regardless of how payments are processed by apps.