The bash scripts invoke programs and wrapper programs. It's a bit ridiculous to say that she'll scripting is the
only thing that operates startup of services in a system.
It's not like new services and startup programs aren't constantly being introduced - so whilst the old scripts were pretty steady, new ones weren't always foolproof.
I'm not adverse to SysV and rc scripts, but they were never the panacea that some grey-beards make out. Dependency orders and parallelisation are particularly difficult, but it could be argued the scripts give control during the boot process.
I think in practice, it's fairly rate that the scripts were really needed - certainly systemd's more declarative nature for a problem that naturally lends itself to a dependency structure that has a directed acyclic graph seems like it's quite an effective an elegant approach!
Aside from the reasonable daemon-arguments issue you raise in the other comment (I just read this, interesting observation!) I really feel that systemd largely eliminates huge quantities of shell scripting in the startup process!
I've been wondering what scenarios there are that would require scripting. I thought that perhaps a more complex setup might be that a service needs to wait on two or three processes before starting, in which case a child can have multiple parents? Can systemd handle that?