The people who matter, who write init scripts, in other words the distro maintainers, were happy to switch.
Why else do you think so many distributions switched?
Yes, the resistance is noisy and stubborn Unix neckbeards. Not even Unix, since every other Unix had something similar to systemd already. LINUX neckbeards.
But you are right, adoption is not enthusiastic, which to me is a massive indictment of the design and usability. We'll basically spin wheels until someone gets annoyed with it and does systemd better, or at least more modularized.
My complaint? sudo systemctl <verb> <service> means the verb cannot be autocompleted or introspected like sudo service <service> <verb> could be. May be minor, but it's generally my only interaction with systemd versus init.d, and to me they completely blew the only thing I use. Not a good impression.
I understand that init.d was a cobbled set of scripts, loose conventions, and even some hacks. But the resistance to system.d is so pervasive it cannot just be stubborn unix neckbeards.