"systemd" is the init system, therefore anything that has it in the name is part of that init system. See also journald, which nobody mentions as being part of the init system even though it's more tied to it (is it even possible to have systemd without journald?)
If they want to change that perception, they will need to change the name (and hopefully stick a stable ABI around it, but that's unlikely to ever happen).
Entirely "optional" yet it runs by default on Arch Linux Arm. I didn't even bother disabling it because it was a lot easier to install Void Linux instead.
The DNS resolver does not belong in the init system or part of it. You just need a working resolv.conf.
It's. Not. No idea why people keep repeating this. The DNS resolver belongs to systemd-resolved, is entirely optional and off by default. Nothing about it is in the init system.
And maybe you don't need socket activation, but there are plenty of valid use cases for it.
See: http://0pointer.de/blog/projects/socket-activation.html