It has already happened on android in the case of Epic. They require that you side load their launcher/store in order to play games such as Fortnite. There was a big lawsuit about google’s fees which precipitated this.

https://www.fortnite.com/mobile/android/new-device?lang=en-U...

My hypothesis is that it’ll happen to both iOS and Android once iOS has been forced to allow third party app stores in most major markets.

Why? Because historically, iOS has been the more profitable platform for mobile developers by a large margin. The payoff for building and maintaining an alternative app store for Android only is questionable, but improves significantly if the store can exist on iOS too.