I'm pretty sure that they want to have init freedom in other distros too. Of course it's possible to fork every distro on any decision but it would probably be nicer to have more options in one distro than many single-option distros.
While it is possible to uninstall systemd in debian at least a few rather important packages depend on it (e.g. Xorg: https://packages.debian.org/buster/xserver-xorg-core ; same for Arch btw: https://www.archlinux.org/packages/extra/x86_64/xorg-server/) which makes this procedure more work for users...
(Considering that enough people seem to have time to create forks like devuan I can't really imagine that the additional time for maintenance would have been a problem for e.g. debian.)
Since there now seem to be a few distros that keep working pretty well without systemd since a few years init freedom in mainstream distros is less important I'd guess but it would still be preferable.
Oh, woops; interesting, I didn't know that... (it seems to be able to handle System V init scripts too though: http://www.slackware.com/config/init.php ("System V Compatibility"))
I had a look at the FreeBSD documentation for init scripts; if I understood it correctly your script doesn't look temporary at all then :-)