> If phones were more open, we'd be free to mod our own games and back up our own save files, but we'd have to live with every random crap app we download being able to take over the whole machine permanently any time.

It's possible to give the user full control and the ability to give apps more permissions, while still sandboxing apps by default. Consider, for example, flatpak, which can make the host filesystem totally invisible to apps but also allows the user to pass in files via the file picker or to totally expose paths by overriding its filesystem context.