I don't think it's as clean cut as non gnome/kde DEs are stagnant. Those two are the main ones in terms of contributors and funding. The other X11 only DEs cater to a specific user group (simplicity and stable) over chase of the whatever. The xfce devs (of which none is employed to just develop xfce, and some are employed at RH to develop Gnome) took a look at wayland, saw what was available, what were their resources and defined a multi-release plan. They didn't break existing installs, neither started developing a linux only login manager.The current KDE is a triumph of style over substance.
A terribly overgrown hole. I remember the KDE2 days...
Then I also used KDE3 and KDE4. KDE5.
KDE3 was quite nice and light...
All simple environments basically don't develop.But large projects swell significantly.
They thought about using CSD but reverted course because they listened to their users and instead gave the option to users and kept the default as it was. These decisions show that the project is mature, that appeals to a kind of users. Others perfer Gnome or Kde because they go all in wayland and systemd because that's the future, or niri or whatever.
As long as the developers are happy with they are producing and I'm happy with what is produced I'm fine. If one ot those changes I'll have to look elsewhere (maybe i'll go back to fluxbox or whatever).