If the GUI stack is vulnerable, then those sandboxes could be broken out of. The idea behind not allowing an app to access root is to remove the attack surface introduced by the GUI stack. An alternative interface to a GUI would be some physical connection (like usb-c). So accessing root exclusively via a console port or USB would be safer in theory.

This is true regardless if it's a phone or a PC.

Desktops are unfortunately waaaay behind something like GrapheneOS or iOS in terms of sandboxing. The closest in the desktop world is Qubes OS, but that's not a realistic alternative to normal OSes for the common user.