Fortunately, I think more projects are abandoning GTK than onboarding it thanks to GTK's horrible documentation, instability, opinionatedness, and simply how much better Qt is to develop for[0], which I can only hope will gradually de-throne GNOME as "the default DE".
It still feels more polished and accessible overall than alternatives though, and so it will probably continue to be “the default DE” for the foreseeable future.
KDE has the most potential to replace it, but I think its configuration is still intimidating (to the point that you regularly see people ask which KDE distros have good defaults), as well as quirks you don’t see anywhere else, like turning file copy dialogs into notification toasts. It’s technically functional but comes off as weird.
The other GTK DEs (XFCE and Cinnamon) I think are in a better place in terms of their settings panes not being scary and generally feeling well designed, but lack resources.
That's a great thing that I'm surprised other DEs don't do. Works really well for file downloads too. It makes intuitive sense and the fact that finished state is retained in notification history is such a helpful thing that genuinely improves user experience for someone with such a poor working memory as mine that it's really weird to see someone calling it weird.
Why do you think it's not practical? That's what Plasma offers already and I'm using it often.
Even if for the sake of argument we accept that Gnome/GTK’s UI choices are poor, the consistency across applications elevates the overall experience.
If, for example, you disagree with the placement of buttons, you know that all the GTK buttons will have the button in the same place which greatly reduces cognitive load.
The same is, unfortunately, not true of other Linux UI kits.
This is vehemently untrue. You know that all GNOME applications will have the buttons in the same place (GNOME writes their own HID) but GTK doesn't have an HID. GTK is simply a GUI toolkit, I can put my topbar at the bottom of my window if I want. It's not 'correct' but I can still do it and ship my application.
GIMP still doesn't have stable release out with GTK3. Inkscape took a long time. I bet both will have great trouble in the future. It seems we're moving towards GTK apps being either core GNOME (maintained as paid work by Red Hat) or trivial little apps like on mobile (either simple to port or just short-lived). Everything else will slowly die out.
E.g. I did not write, but just looked through the hoops PyQt has to jump to interface with Qt nicely. Quite impressive.