Correct, I'm not just talking about alternative OSs. I specifically had application-layer software in mind when I wrote that comment. iOS prevents users from in

Correct, I'm not just talking about alternative OSs. I specifically had application-layer software in mind when I wrote that comment. iOS prevents users from installing applications or app stores not approved (digitally signed) by Apple. Solve that problem, and Apple wouldn't have any leverage to force application developers to pay per-install in the first place.