I guess we should do away with memory protection as well. Filesystem permissions? Bah, they can go too, after all, a computer is generally used by a single person right?

The reality is that many users use untrusted applications that don't have access to home, ergo Flatpak. There are plenty of reasons why the free for all security model for X11 isn't suitable. Besides, that ship has well and truly sailed - most of the X11 devs have been working on Wayland for the better part of a decade now.

For instance, I would argue that "basic functionality" today, established by browsers and phones more than 15 years ago, is the ability to sandbox applications and not treat them as all-powerful. X still doesn't have that today. Wayland had that as a basic design premise from the beginning.