> Do users really want ...

The answer is clearly yes in a lot of markets, even if purpose-built apps might be better at a specific tasks, a single app that does dozens of things "well enough" is more convenient than having to juggle dozens of login infos and para-relationships with app developers (managing graymail etc).

Does't this "the market is right" logic also apply to Apple itself and its walled garden? E.g.: "Do users really want a walled garden where they can't install 'everything apps'? The answer is clearly yes in a lot of markets..."

both things can be true, apple has over a billion users

The extreme result is indeed what Apple wants to avoid because you would more or less have a custom operating system at that point and could ignore Apple's software, which they would hate. Obviously it is not as good as being able to flash an actual new OS onto the device but it would still impact Apple's bottom line.

Google itself is a superapp at this point as you only have one account. But to answer your question, I think it’s because of interoperability issues. Why can’t my calendar services message me? Or why can’t I quickly create an event inside a chat? If you remember PDAs, they fell under the definition of one ecosystem to manage your communication and time, but now you have several services that refuses to talk to each other. One of the core strength of Apple is that kind of integration. It’s not that you want one company managing it all, you just want an integrated app ecosystem.