I have yet to see a convincing argument that this was really necessary from a User prospective. The only argument I will buy is the limitations in what could be on the store like Adult apps, emulation, and similar. Which is valid, but not something requiring a separate store.
The day that an app requires me to install a separate App Store to use is the day my smartphone becomes less useful and I stop using that particular app. Being on the App Store has benefits to me as a User, particularly when it comes to privacy and billing. Do we really need to bring up just how much companies like Facebook pushed back on the app tracking bulletin and the scary popups many apps put up to try to convince you to allow them to track you?
Or the dark patterns that many companies continue to engage in regarding billing? Right now we are dealing with Cable companies complaining about yearly notices of renewal and being able to easily cancel, and you are going to try to tell me that companies won't find a way to abuse this?
Literally today I was trying to cancel a task management app that I mistakenly signed up on the web instead of through the app, and I have to email them to do so. But they are more than happy to let me increase the cost of my subscription with a single click.
So no this is not about choice for Users as this continues to be framed as, my choice was to go with an iOS Device in the first place for this walled garden. In the coming years the choice will be more and more made for me by developers. There is basically zero chance that Facebook, maybe Amazon, and companies big enough to try it won't jump at this chance when they have a big enough name to be able to leave the App Store and other developers can just piggy back on that.