Um, the fact I've told any semi-non-competent person to buy iPhone because it very specifically won't screw up like the popup ridden land of Android.
How does this relate to your original comment claiming you're losing something you paid for?
You know that developers will end up using alternate App Stores.
Which means I will be forced to use them if I want to continue to use the app.
This keeps being stated as though it’s a foregone conclusion. All evidence points to the contrary. Look at Google Play. Android has allowed third-party stores since inception and yet the only stores that see any notable use are the ones that ship on devices, i.e. Google Play and to a much smaller extent OEM stores (at least in the English market).
So no, it is not likely that you will have to install another store. Just like most Android users only ever touch the Play Store most iOS users will only ever touch the App Store.
The only notable stores I expect to see from this are an Epic Games store that basically just has Fortnite on it, and some community-driven F-Droid-like.