Just download from the App Store. Nobody is forcing you to use the Facebook store or whatever.
Plus you scenario is purely a figment of your imagination. Does any other operating system have the problem you described?
One of the benefits of the App Store is the restrictions that Apple puts on apps it allows to be sold. As a consumer I benefit from those restrictions. Right now, apps have no choice if they want to be in the App Store. Where the world you want to come to pass (a situation where every one can install anything from anywhere, many apps that now live with those restrictions would move to an store or installation method that that did not have them.
That would harm me.
In addition, I like that I only have one source for all apps, one place to go to do upgrades, restoring my device is simple and fast because of that.
Your argument seems to be that no company would be in another App Store, so I would not have to deal with it, but my life would be better if developers had choice.
I agree that my life would be different, and somethings might be better for some people, but most things would be worse for me and those who share my views.
Do you have to download Facebook from the Facebook Store on Android?
No, but piracy is a serious problem on Android because of side loading, and it is not on iOS. Again, this benefits me as a consumer. I am glad there is another platform that offers substantially the same functionality as iOS, but with different choices around privacy, tracking, permissions,
etc.
More than enough competition in the market without needing to destroy iOS/iPadOS/tvOS/WatchOS to "increase" it.