I think you may be missing part of what the GP is saying: the concern isn't (primarily) that an alternative App Store will overall become more popular than Apple's. It's that specific apps like Facebook will create their own App Stores whose primary purpose is to distribute their one or small number of apps without the restrictions Apple places on privacy.

So the Facebook app (and Instagram, WhatsApp, and whatever else Facebook owns these days) would be able to collect as much data as the OS itself allows, without any kind of warnings before install.

It could potentially even use private APIs of some sort to bypass Apple's OS-level permissions dialogs and collect data without even asking the user first—it's unclear, at this point, to what extent Apple would be able to police this sort of behavior from motivated bad actors like Facebook when they're not being distributed through Apple's App Store.