Perhaps, but Apple would still have an edge on tight integration with the OS resulting in a quick and frictionless user experience. If that wouldn't cut it for some developers, so they would bother themselves building a separate processing, well, then Apple would probably have to lower their fees.

Personally for me, as a developer of applications, I could care less about 30% fee. I find it reasonable. What I DO care about is restrictions on owning the device and users' inability to run any app they need - even if it is not vetted by Apple. In part it is because I happen to live in an authoritarian country which government loves to block apps in the AppStore.

No, Android is an existence proof that this will not happen. Users in fact hate it, so apps mostly stick to in app purchases.

Only because of the 30% cut. Very few apps would start doing it if the App Store took a cut similar to what Stripe takes.

Exactly. Apple's market power derives from the consumers that trust it to represent them in negotiations with publishers. The decision to "not give every app your credit card" is made by consumers when they purchase an iPhone.

Purchasing an iPhone is an act of collective bargaining by consumers against publishers that otherwise would hoover up their private data. "If you don't like it, you can always live like it's 2004, before smart phones" is not an answer iPhone customers accept.

That's not how it works, apps follow what customers want - Android exists and apps go trough play store and Google payment.