The ability to replace a module with an identical module is not actually modularity. What If I don't want to touch dbus with a 10 foot pole? How about if I want SysV in charge but call systemd for a set of subsystems? SysV is modular enough to deal with that, is systemd?
The scripts aren't generally anything like complex in SysV. They're mostly all the same, and so quite easy to quickly understand.
Systemd doesn't understand imperitives. Sometimes I want the system to just shut up and run the command I say. I don't want to be second guessed.
Systemd is easy to use so long as you happen to want exactly what it wants to do. If you want anything else, it is somewhere between much harder than SysV and impossible.
Have you ever tried to get systemd to mount a btrfs root filesystem in degraded mode? Good luck with that!