> the concern isn't that users will jump ship from the App Store [...] but that developers will
What is the practical difference to you, as a user, between:
A) Not being able to install Fortnite because it's only available on a third-party iOS storefront,
and
B) Not being able to install Fortnite because it's not available on iOS.
If apps jump ship from the official store, you personally as a security-conscious user won't be able to install them. But if apps jump ship from iOS, you also won't be able to install them. So who cares if developers move off the official iOS store? Aren't they already free to do so today?
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I'm going to go out on a limb and say the difference is that (deep down) we all know that Epic is right, and Apple really is one half of a duopoly -- a half that controls over 50% of the entire mobile app store revenue in the US -- and that it would be insane for an app like Fortnite to drop iOS. I think the difference between the scenarios above is that (deep down) both you and I know that Fortnite isn't really free to abandon iOS as a platform, and that the stranglehold iOS has over the market is the only reason apps like Fortnite are on iOS in the first place.
We know that given the choice, consumers and developers would both choose a more open device. And we know that the only reason the closed ecosystem works at all is because many developers and users don't have a realistic choice about whether or not to accept Apple's terms.
The reason people see the two scenarios I list above as different is because, yeah, all of us on HN do actually know that Fortnite doesn't really have the option to walk away from Apple devices, and without a 3rd-party store Epic will be forced to agree to pretty much any terms that Apple requires -- they have no negotiating power. And once we admit that, then it becomes a lot more obvious why developers are asking for some kind of regulation around app store policies.
If dropping iOS and supporting only Android or PC was actually a realistic, sufficient option for most developers, then you wouldn't be worried that they'd all jump ship the moment they had a 3rd-party store as an option on iOS -- those developers would have already left the Apple app store (and iOS) behind.