>On my Debian system, /etc/init.d/ssh is 167 lines of almost entirely boilerplate shell script that has to be maintained separately for each service

That's a failure on debian's part, not a fundamental flaw of init. Guess what the equivalent looks like on openbsd?

    daemon="/usr/sbin/sshd" 
                        
    . /etc/rc.d/rc.subr     
                        
    rc_cmd $1