Do you think Meta is going to hang around on the App Store? As soon as they can sidestep Apple's privacy protections, they'll do it.
Yes, Meta is going to hang around, as you have two scenarios:
[1] iPhone comes with the AppStore, you download the Meta App that is already available. This is quick and easy, as it's one easy step.
[2] iPhone comes with the AppStore, you can't find the Meta App, Meta is telling you to download another store, you need to sign up first. You sign up (maybe you can use your Meta account), download the store, download the Meta app. This is hard, as it's as it requires more than two steps, three, four if you look for the app in the AppStore and the average user will be already frustrated.
Since it's already established, that you get more success with less clicks (why do you think you can book a hotel directly from google maps, or why those encompassing pages like grubhub are succesful, it's because they are convenient and quick to use) Meta might offer their App on a different store, but will likely hang around the AppStore, because this is how they can entice new users to quickly install their App.
You can doomsday scenario all you want, but most of the time, the Apps will remain on the stores.