But if it starts becoming a required component for turning up the volume, that is clearly a sign of poor design.
Systemd's integration into more than just init is a fundamental result of a singular problem that has been facing all operating systems for about 15 years now: Hot plugging. Hot plugged devices need to be handled in almost exactly the same way as non-hot swapped devices during boot, so it makes sense to use the same code path for both processes. This effectively means that your init system needs to also handle pretty much every type of device that can be hot plugged. This includes: any and all USB devices, large parts of the audio sub-system, network devices, damn near everything these days. By definition, the software that handles this needs to underly everything except the kernel. Since the kernel does not deal with this (by design), something else has to. Prior to SystemD, the various methods for handling it were a complex jumble of incompatible broken-ness.