If only Apple could, say, permit payments outside of the Apple store for apps installed from the store...

This was the real problem. Apple has to get a cut if you run the app on an Apple-made device and either sell the software or charge money for digital goods at all.

There is only upside for Apple with these restrictions, so I see them continuing to fight this fight with the excuse being "Customer experience, security concerns," etc, where the reality is maintaining the wall and gate in their garden.