Most people dont like too many choices. And sideloading just might destroy the experience for the majority, because many big players could choose to have their own store.
Sideloading didn't destroy MacOS, Windows or Android. Unless you have evidence that iOS is somehow different, I'm going to assume that it won't destroy iOS either.
Phones are different from computers, it is a different market with different requirements. Apple app store is much more succesful than Android “app store”. That is the evidens for you.