Theres the really deeply embedded systems running on tiny processors which probably only run a single binary and are never updated.

And then there are big systems (think cellphones, automotive infotainment, etc.) running on quad core procoessors where you have dozens of processes, which might even be independently installed or updated. For those kind of systems you really want a sophisticated init system, and the cost of systemd is probably minimal compared to the instances of chromium/webkit/blink that you might already have on your system.