You mean union mounts? Typically doing a union mount is a privileged operation, while setting $PATH is not, so you cannot really replace the latter with the former. A bigger issue is that, even if mounting various things on /bin were unprivileged, removing $PATH from Linux would break an enormous amount of existing software. Similarly with file permissions. So union mounts don't permit you to simplify Linux in that way, even though they did permit you to simplify Plan9. (Incidentally, Plan9 did have the traditional user/group mode bits.)