> It is relevant, because if it were a free platform you'd be able to do this kind of thing without circumventing the system software.

You can do it without "circumventing the system software." Unlocking the system partition is a supported operation. Then just install a program that lets you run other programs as uid 0. This is not possible for normal apps to do because running as uid 0 means you are no longer subject to the permissions system.

Also, it's irrelevant because the whole point is that Google can't block the user from running whatever apps they like by blocking the app from the Play Store. Requiring root access is orthogonal to Play Store availability.

I don't need to use third-party software to enable UAC in Windows or sudo in Linux.

Their security model is different. They are multiuser OSes protecting users against each other, while Android started as a single user OS (despite what goes on with uids underneath) protecting users from themselves.

Users running their own apps as uid 0 goes directly against the security model. Nothing stops them from changing or completely discarding the security model on their own devices, but once again, that is completely orthogonal to blocking apps on the Play Store.

It's not completely orthogonal, though. Both things are, in practice, means by which Google can control what kind of apps most users will be exposed to and will install.