You seem to be forgetting about all of the other unixes. Because systemd does not (to my knowledge) support any unix other than Linux, we can say for certain t

You seem to be forgetting about all of the other unixes. Because systemd does not (to my knowledge) support any unix other than Linux, we can say for certain that OpenBSD, FreeBSD, NetBSD, MINIX 3, Plan 9, GNU/Hurd, MAC OSX, et al. all do not use systemd. Add to that the fact that some Linux distributions do not use systemd (Including Slackware, etc), and you can easily come to the conclusion that systemd is (strictly speaking) in the minority, and therefore not the main unix init system these days at all.