That right there is my problem with this. We can't even definitively say that it isn't possible. Doesn't Epic do it on Android?
A big enough company (like Facebook, Microsoft, Amazon) could easily push their store as being the primary way to download their apps. Start out on both at first but gradually introduce incentives for the users to redownload somewhere else. Then suddenly that App Store is on their phone, other developers see the App Store being used a lot and more and more developers stop using the App Store because they can get around these protections that I get as a user.
If somehow developers were still required to be on the App Store, fine. I would have no issue with it. But I just don't see developers doing it. Too many already try to employ dark patterns on there website that given a chance there is no way they won't try to do them on mobile. Many already try (like with the prompts to permit app tracking) but are limited in what they can do.