I suspect many sysadmins, certainly the ones who complain about syslog, were quite happy with init.d

Traditionally developers would write code and packagers would package them into a distro specific rpm/deb -- including the init scripts (which may be pushed back upstream). Developers wanted to bypass this slow process, and there were far more developers than people willing to package the software.

Personally I've never had a problem writing an init.d script.

All I ever could do was add a line to rc.local that started a supervisor process to run my stuff. But I have many service unit’s as part of user-data.sh or cloud init and they work and restart on failure and are visible to other admins I haven’t talked to etc. it was surprisingly easy.