You missed the "I should be able to" part of the sentence.

Then maybe they should not operate the app store at all if it's incompatible with user choices. Or at least, leave the user a choice to just download an app elsewhere if they want to.

Clicking on a link and installing an app like you would do on windows seems the right thing to do.

How is that even relevant? The parent was specifically saying that you should be able to sideload apps so that even if an app was removed from the official store, you could still install it.

Oh yeah, Android is also completely broken by design, I'm not advocating for the way it works there either.

Every android phone I've owned can install apks just by opening them in the file manager. Not all Android phones allow this, but many do.

On top of that many brands (asus, oneplus, even some Chinese brands) let you unlock the bootloader with official tools. Using this you can take full control.

So it's far from 'none'. I recently bought a zenfone 6 because it satisfies this criterion. Asus is even sending devices to alternative os developers to help them out. So we can choose to not support walled gardens, and it's not even hard. It's a question of getting people to value software freedom higher.