In 5 years, all userspace applications that don't get merged into the kernel will have been swallowed by systemd instead.
But seriously, why doesn't anyone mention binder in this context? It's already in staging, last I checked. Binder is a kernel interface for what is widely viewed as a simpler, lighter weight replacement for D-Bus. Binder's native communication is pub-sub, with userspace programs registering for events for which they define handlers. Assuredly pub-sub and multicast have some similarities, although I don't know if binder (which is intended for a fairly narrow set of use cases) will scale well, since scalability along with speed appear to be the main goals of AF_DBUS.