But why would they do so? That makes no objective sense.

Systemd never was "merely" only an init system. And it makes no sense for init systems to grow to systemd-size either, in order to solve non-init related issues.

> In the case of GNOME, KDE etc depending on it, the reason mainly boils down to "we could implement our own manager for handling desktop daemons etc or just get systemd to do it for us"

That's not quite true. GNOME always was close to systemd devs due to funding. KDE was less close, but even within KDE some people lobbied for it such as dave edmunson or however you spell the name, and "me-needs-a-donate-daemon" Nate, who you are not allowed to critisize on #kde reddit. But I agree that they could simplify some code by depending on systemd. Of course this now means that KDE is sold in a dead-lock with systemd. I wonder if I can still use konsole without systemd. I tend to use iceWM since it is so much faster than KDE or GNOME, but when konsole depends on systemd I may indeed need to switch to another terminal. That will be painful though, but there is no stopping systemd - it infects and taints.