Unix domain sockets or FIFOs could be an alternative, but you'd have to do all kinds of complexity to support pub/sub style many to many communication over them (which is what dbus effectively does for you).
Once you hide the complexities of using sockets you just end up with some sort of message bus that each application connects to and registers an ID with... which is essentially what dbus is.
Anyway, all the complexities of using sockets/FIFOs could be hidden inside libraries.