Have you actually read the mail you're giving as "evidence"?
The last paragraph begins with: "The current idea is that systemd will provide a bridge service, that
offers the current D-Bus socket, and an unmodified libdbus (or an
alternative implementation) can talk to that socket like it talks
today to the dbus-daemon."
kdbus is also not a NIH-implementation, in any meaning of the word. It was "invented there", and it also fixes things: it will have much lower latency and overhead than the current userspace D-Bus.
Also, technically developers don't have to use systemd's dbus library; apparently Gnome's doing direct calls to the kdbus kernel API instead. That makes it even harder to support systems that don't have kdbus+systemd, of course, but this is Gnome we're talking about.
Umm... the mere existence of the compatibility daemon makes it pretty clear that one could easily build a system which interfaced with kdbus without changing much of your existing code at all. Your code wouldn't have to be that modular to pull it off.
Honestly, the Linux kernel has, for the longest time kind of been filled with these ad-hoc, efficient IPC mechanisms like netlink. It has severely needed SOMETHING like kdbus, and you can see the key pain points in Linux have already been addressed by other systems using their own proprietary or semi-proprietary mechanism (which invariably happens if you are late to a party that people need addressed immediately).