If systemd were just a replacement init, you'd be right. It's not just a replacement init. A replacement init would stop and start system services and do nothing else.
There were multiple projects to develop new inits. They were coming along nicely, until poettering decided he had to control everything in userspace and used his position at red hat as leverage to shut them out.
Systemd currently incorporates device management, DNS, dhcp, cron, hostname provision, network management, system logging, interprocess communication, ntp, user session management, container management, user home directory provision, and will shortly absorb the management of the entire boot process. All of its components are intermingled and non-trivially linked, requiring shims and compatibility layers when they aren't present, and it has infiltrated itself into so many aspects of userspace that it is a required dependency of more than one window manager. Gnome requires it. It is an enormous, bloated, hog of a system that brings none of the benefits it claims, whilst also creating a whole host of new, system-wide attack surfaces.
As for pulse: it's a bug-ridden, overly-complicated wrapper around ALSA, which poettering created entirely because he couldn't figure out how to multiplex audio to dynamic soundcards, and couldn't be arsed to ask.