KWin is not designed to run as root. It is not hardened enough and
there is a great risk that applications could attack KWin through
X11 properties, Wayland protocol requests, etc. to trigger stack or
heap overflows and execute random code. As clients connected to KWin
could be remote there is a great risk in running KWin as root. Also
clients on the same system but started as a different user could
try to gain more privs by exploiting KWin.
Furthermore KWin is designed to not run as root. It interacts with
logind to get the devices opened which would require root. Whether
KWin would work as root at all is questionable.
We cannot guarantee that running KWin as root is secure, thus this
change disallows running KWin and thus a Wayland session as root.