> Is there any reason that Xorg couldn't tack on a permission system, other than that it would be inelegant?

That's the whole reason for Wayland's existence: such things were not "easily" possible...

Linux+Xorg desktop would be hopelessly insecure for ever because these security features were deemed too hard to the point of not being "worth it". So Wayland was started.

Someone is developing XLibre, and Xorg fork. It may be pulled off, but I doubt it. Making Xorg safe was tried many times for many safety-holes.