How does forking Debian solve anything? The issue is that Gnome, KDE, and other software requires systemd. You need to fix/maintain their compatibility with other init systems. Then it is easy for Debian user to switch init systems.
This I can't wrap my head around, why does Gnome for example need systemd? There are standard ways on GNU/Linux to do everything it needs in a unix fashion (by deferring to small specialized tools):
shutdown
reboot
whoami
uname
mount
...
These and similar tools (or libc functions) give you all information you need (username, etc.) and system functions a desktop environment needs (reboot, hibernate). You then only need something like udev for plug-and-play (which is available without systemd, and as a part of it), and interfaces to the X/Wayland server and the audio system (which are both not a part of systemd). This seems to be all stuff that can easily be abstracted away in one module per OS. I don't see why you have to tightly couple your DE to systemd. (Unless, you are using your DE as a vehicle to push systemd, which I hope is not the case...)