GNOME and/or systemd

It's a bit misleading to say that making systemd a dependency for user sessions inherently requires using it for boot or the base system. It should be possible to boot with Upstart, SysV init, or any system of your choice and still run "systemd --user" to manage the user session. I doubt this will ever be a first-class option, but we shouldn't prematurely rule it out.

Of course, if a utility running in the user session interacts with the core OS to *manage* user sessions and expects to find systemd there, running systemd in the user session alone won't be enough. That makes the power plugin different from other parts of GNOME that could use a user session instance of systemd for, say, launching applications and session services like PulseAudio.