In fact, systemd is arguable a more important component covering more aspects of system function in a lot of distributions: home mounting, boot process, logging, init, cron... I'm going to start calling it systemd/Linux, just to mess with Stallman.
Uh no, it's not. GNU has been integral to the GNU/Linux project for years. Without GCC, coreutils, glibc, there would be no linux distributions. Systemd has not played the same role.
systemd is an init system and just has not played the same role in the development of GNU/Linux distributions like GNU has. before systemd there was sysvinit, and there are number of alternate init systems. It's not about system functionality that we name operating systems.