Well, to clarify, you also don't have to redirect its output to a different log daemon. You could totally have logging disabled, in the sense that it's not output anywhere.

I do not know if that means that journald is turned off per se. However, because I assume that Lennart and Co. are not total amateur programmers, I feel like it's safe to assume that if logging is disabled and there is no other logging daemon enabled, than the relevant code paths are not used. There must be some other reason that journald is required.

My guess is- and again, I know pretty much nothing about systemd's code- that journald is also holding in memory information about the current state of the system, so that systemd can react accordingly. They aren't going to force a dependency unless they think it's necessary, so I can only assume that journald does something necessary for systemd to do its job, such as "logging" what the daemons in your system are doing, so that systemd can restart or launch daemons when appropriate.

I honestly think that people's psychology is to blame for some of these complaints. I bet if journald was just rolled into systemd from the start and never given a name, nobody would complain. You'd just say "systemd keeps track of the daemons and their status and can react appropriately to changes in the system state. Systemd also allows to you enable/disable logging in its own log format, or just using one of the old tried-and-true logging daemons." People would love it. They'd laud systemd's ability to initialize and monitor and fix the running system. But as soon as you give the logging a separate name it turns into "HOW DO I REMOVE THE BLOAT?!"

I bet if Firefox had a dependency called "Firefox-URL-bar" people would be arguing about why you should be able to run a web browser without a URL bar. Granted, nobody actually wants to run without a URL bar, but they think it should be an option. That Firefox is forcing you to use their URL bar hard dependency is bloat.

That's just my cynical view of the situation. Like I admitted several times, I'm pretty ignorant of how systemd really works deep down, and until someone like Lennart tells me what journald does when you disable all log output, I really can't assume that it's necessary or not.