Unpacking greetd (0.9.0-3) ... Setting up greetd (0.9.0-3) ... Failed to preset unit, file "/etc/systemd/system/display-manager.service" already exists and is a symlink to "/lib/systemd/system/xdm.service". /usr/bin/deb-systemd-helper: error: systemctl preset failed on greetd.service: No such file or directory
AFAICT this is actually https://bugs.debian.org/1023982, but this/your bug shows the reason why an/the alternatives system should be used. If you agree it's the same bug, please merge them.