Nothing Apple does is a natural monopoly.

Artificial monopolies can be maintained through vertical integration. If you don't like their app store, use another one. If all they provide is the app store, that's easy, and every platform would have half a dozen or more.

But if using a different app store requires you to develop your own operating system, and convince millions of third party developers to make applications for it even before it has any users, and make your own hardware, on and on, well then it's infeasible to make a competing app store and you get an uncompetitive and abusive market.

The solution there isn't to regulate it like a utility, it's to break up the company so that the operating system isn't made by the same company as the app store and has no reason to restrict the users to only the one. Then you have competition again and don't need any regulation other than the constraint on vertical integration.