Except, in case of iOS AppStore, you don't have the choice of a third party store or even side-loading ability. And they are in the process in doing so (partly

Except, in case of iOS AppStore, you don't have the choice of a third party store or even side-loading ability. And they are in the process in doing so (partly by closing the platform through read-only system partition and expecting the app developer to sign the app through Apple means) with MacOS. The platform is not yet closed, as the system partition can still be mounted in read-write mode and notarization can be bypassed for now. But their are clearly in the process to make these changes permanent in near future. Reason why they introduced them in Catalina.