A message bus won’t magically remove the need for developers to sit down together and agree on how some API would work. And not having a message bus also doesn’t magically prevent you from allowing for alternative implementations. Pipewire is an alternative implementation of pulseaudio, and neither of those rely on dbus (pulse can optionally use dbus, but not for its core features). When using dbus, developers have to agree on which path the service owns and which methods it exposes. When using unix sockets, they have to agree where the socket lives and what data format it uses. It’s all the same.
It can even start the receiving daemon if it is not yet running.
We have a tool for that, it’s called an init system. Init systems offer a large degree of control over daemons (centralized logging? making sure things are started in the correct order? letting the user disable and enable different daemons?). Dbus’ autostart mechanism is a poor substitute. Want to run daemons per-user instead of as root? Many init systems let you do that too (I know systemd and runit do).