[vezzy-fnord]

systemd's modules are interdependent and all require systemd to be PID1 in order to function. You cannot, e.g. strip out logind only and use it as a ConsoleKit replacement. Contrast this with a toolkit based approach like s6, which can run as init, process supervisor or both, and can have its tools that operate on process state be used independently.

Another criteria is whether you can swap out one component without breaking the whole. You can't do this for journald, for example. You're forced to keep it by reducing it to a sink that redirects to your syslogd of choice. On the other hand, replacing parts of the GNU coreutils with those of 9base or sbase, for instance, won't break the rest (though it will break programs that depend on the GNU coreutils' extended options, of course).

[MertsA]

The journal is a bit of a contrived example though, it is pretty much the only mandatory part of systemd outside of pid 1 and you can still replace it, you just need something to fill the place of it and no one wants to put forth the effort to create some journald compatible rsyslog daemon because it would be mostly pointless. Also, if you really hate it you can still configure it to not even store run time logs by throwing Storage=None into the config file.