Different visions

Different visions

Perhaps the problem is that there seems to be two different visions as to PID 1's role.

Indeed. Except it's not two “visions”. It's “wishfull thinking” vs “reality”.

For some people, PID 1 is the "initialization daemon", which loads the rest of the system and then stays out of the way.

Yup. Simple. Easy to understand. Impossible to implement on UNIX-like system including Linux.

For other people, PID 1 is the "system manager", which is responsible for managing the whole system.

In UNIX, for better or for worse, PID 1 is “system manager” because it's “parent of all orphans”. That's why initial vision of init included such management. Remember inittab? Unfortunately it was found limiting and instead of fining it people started introducing kludges upon kludges upon kludges.

To these people, PID 1 being on the udev system bus and managing control groups for other processes makes complete sense.

Yup. These are modern day replacements to venerable /etc/inittab and HUP signal. Nothing more, nothing less.

Systemd includes other components, too, but usually when these componets are simpler than interface needed to put them in separate programs.