Point of interest,
After recent discussion, and after reading https://doc.opensuse.org/release-notes/x86_64/openSUSE/Leap/15.0/
5.2 Use update-alternatives to Set Login Manager and Desktop Session
the question of "Why use update-alternatives?" for this configuration functionality instead of "Standardizing around systemd service?"
What benefit does a suse-specific 'yast2-alternatives' package have over standardizing with systemd?
(nit/note: release notes html format should be tweaked to allow selecting heading lines without "\nReport Bug\n#" getting tagged on)