Wouldn’t this also be possible with plain sockets tho? To continue with your example of music players, the current standard is MPRIS, which uses dbus. But in an alternate universe, the people behind MPRIS could just have decided that music players shall create sockets at /run/user/1000/mpris/[player name] that all speak the same standardized protocol. If a player wanted to add functionality beyond MPRIS, it could accept nonstandard requests on its socket, or create a new socket altogether for extended control.
I just don’t see how this would require any more coordination between developers than the current solution. And I don’t see how dbus can save you from having to “understanding every single […] app out there”. If anything, it adds the overhead of learning how dbus itself works, on top of how a specific app’s dbus interface works.