Well, the display manager also launches Xorg as root:

root      2109  1.7  1.2 469952 99548 tty7     Ssl+ 07:52   2:31 /usr/lib/xorg/Xorg -nolisten tcp -auth /var/run/slim.auth vt07

Or am I wrong?

Btw: Is it intrinsic to systemd to run Xorg as user?

Is there any other solution as of now?