Aside from host the infrastructure that delivers the apps. Are they doing this to make money? Of course. But saying they haven't earned it is flat out wrong.
* Maintains a user database including authentication, plus all the support costs (password support is very high)
* Payment handling (no one does it for free) including keeping up with tax authorities and legal systems in much of the world
* Almost unlimited (re)downloads for users to a reasonable number of devices. Bandwidth is cheap but not free.
* Backup and restore for application data. (That functionality is available to apps without extra fees.)
* A curated walled garden including a review process. Apple keeping the dregs out and avoiding the place turning into a cesspool means users can be more confident about the apps and that halo effect helps all apps in the store.
* Mechanisms to extend your app such as IAP, and an advertising solution etc
* Access to a large user base, with reasonably fair rules that everyone has play by
* They make all the above work together
It is certainly worth more than zero percent!
I switched from Android to iOS and I just don't see this at all. Yes I'm more confident installing from the app store because of the security model, but android is getting that too. I'm no more confident that an app or particularly game will be good or worth downloading or buying though, and I don't perceive apps to be high quality in general.
Under that logic they should charge for upgrades. But subscriptions is not really related to upgrades. Subscription implies that your app is tied to your own backend service that you keep up and running without Apple's help.
You could be using iCloud as your backend. You're definitely using their subscription billing infrastructure. You're getting access to their users credit cards without having to ask for them.