It occurs to me that sendmsg() is already kind of a standard, and the problem of drop in replacements could be solved by just making the replacement bind to the same file path and emulate the same protocol, and the problem of automatically starting the daemon could be handled by a systemd socket (or even inetd if you wanna go old school). The only advantage that I can see dbus really having over Unix sockets is allowing multiple programs to respond to the same message, which is a definite advantage but AFAIK not many things take advantage of that.