> Furthermore, any big vendor (e.g. Facebook) with its own vast audience and ecosystem would immediately launch its own app store

And yet there is no precedent for this happening, because this didn't happen on Android, Windows or macOS. The Play Store shows the same scary privacy warnings the App Store does for Facebook, and there still isn't a Facebook app store that some people like to fearmonger about.

The Play Store cannot be compared to the iOS App Store, the Windows Store, or the macOS App Store because the iOS App Store is substantially more stringent than the Play Store in what apps are even allowed to request permission for, their sandboxing restrictions, and their privacy requirements. For example, Google gets more than 10X the data from their Android app than they get from their iOS app according to estimates. Also, Google Play only shows a request for Permissions, while iOS shows the entire list of what data is collected that is linked to you, and the data that is collected that is not linked to you, which means that the "scary privacy warnings" are substantially scarier on iOS and don't bear the comparison to the permissions-only Play Store.

If they can’t make a Facebook App Store in Android with its more open platform, how much more for iOS?

What... the Epic store is the BIGGEST example of this. It is a shoddy game store compared to Steam and GOG. Epic paid for massive exclusives to make inroads into the PC gaming space and their product is full of dark patterns.

Others have mentioned Steam, and the lack of an equivalent to Apple’s latest tracking restrictions on Android.

But at the risk of pointing out the obvious: on Windows and macOS, there is no Facebook app at all. There’s a website, because somehow websites have become the standard for desktop applications, even as they’re simultaneously seen as unacceptable on mobile. And Facebook has full control over their website.

Of companies that do make native desktop apps, the big ones almost all eschew the Mac App Store.

Steam?

I think the best example is... Epic Store itself. It is full of overpaid-for exclusives and has a huge feature disparity with Steam

Steam is older, so it's had time to grow the feature set that it needs, but it's not fundamentally different. Valve Software was a company that released games, before branching out into the much more profitable business of intermediating.

I'd argue that Epic has the opportunity see what features are out there, what is possible and what users want and could imitate Steam much quicker than it would've taken Steam to build the functionality originally.

Afterall, it was released in 2018.

And where does that end? Google's got a bunch of apps like Gmail and Google Drive, so the Google Play Store for iOS would probably enter the ring too.

The point is more that if the App Store is to be the only App Store on iOS, and Apple wants to maintain its rules on privacy and other systems and not become Android, sideloading logically cannot be permitted.

Sounds like a proper marketplace.

Not really—with a marketplace, you can choose to buy a product from Store X, or from Store Y. In this future, you'll still have no choice which store you're buying a product from. Apps like Facebook aren't fungible. And if you want Facebook, you'll have to buy it from the Facebook Store, whether you want to or not.

…for publishers. Stores have to compete for software to publish but users don’t get any additional choice when Facebook is only available on the Facebook App Marketplace.

This is a little like exclusives at physical stores but customers have a much less involved relationship in that case.