…systemd very much does use the init system to launch userland and GUI processes. That’s how GNOME works.
Dbus is for interprocess communication. The fact that its primary use case is communication between desktop applications is hardly relevant to its design. I don’t see how GUI frameworks are at all relevant, or how it would be possible to create an interprocess communication mechanism that only worked with one GUI framework without some heroic levels of abstraction violation (which I would not put past Qt, but that’s another story).
I don’t see why having an entire dbus daemon running in the background is better than having a cluttered /tmp or /run directory.