Okay, correction/clarification: it requires everyone to rewrite their code if they want to use kdbus rather than doing old-style dbus calls to the compatibility daemon, which may not be installed by default and is a fairly ugly hack that's probably going away in the future. It needs a bunch of special support code in systemd that's not used for anything else, and the generall expectation seems to be it's a stopgap and most or all dbus consumers will move to kdbus.
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.
> it requires everyone to rewrite their code if they want to use kdbus rather than doing old-style dbus calls to the compatibility daemon
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).
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.