The problem will be if some large developer - or maybe a small developer of a suddenly hot app - decides to only allow 3rd-party payment, with no Apple-sourced payment accepted. Then people will be faced with the choice of "use this necessary and/or desirable app, and give your details to some third party payment system", or stay in the walled garden and not use the app. And if it's, say, something you need for work, that could pose a pretty annoying problem.I tend to think most users will bypass the outside payment option, for a number of reasons, including the security in the Apple App Store model, and the simple convenience of using the built-in tools, along with the relative ease of managing a subscription right on the device, in the OS. This maybe more of a Pyrrhic victory for devs seeking to avoid paying apple a cut of subscription fees and the like. You won. Now, make it both cheaper for the user (because if you charge the same amount for whatever content, why would the consumer use your solution) AND at least as convenient.
I have friends/relatives who say they wouldn't pay through some 3rd-party source, as they trust Apple more.
The folks who keep asserting that "my ability to use a 3rd-party app store doesn't affect you, just keep using Apple" don't see to get this argument.