Let's not overreact:
apt-get purge systemd works just fine.
The result is: apt-get will automatically suggest removing systemd and install sysvinit-core
That may work until systemd is a hard dependency for everything on your machine.
this is Debian remember -- it'll work as long as anyone thinks it's worth the effort to make it work.
If the volume of noise made by the anti crowd had any correlation with available effort then that would indicate that it would work forever, but my suspicion is that those people are all mouth and no trousers.