Things like that make me thing this can be arguably the biggest waste of effort of all FOSS development, all because "fear of the new" - Trying to keep the huge monstrosity of Qt3 which EOL was more than 10-15 years ago, the whole DE that was built on it and all of their apps, even developing new ones... Just for the appeal of few people with the nostalgia for that kind of GUIs. One could argue KDE5 is as stable or even more than KDE3 ever was, and less buggy - not even mentioning the ability to replicate that interface with a few clicks and some fiddling.
But whatever makes them happy.
It's also worth noting that people have different workflows and tastes. One of the perks of FLOSS is the ability for people to choose what works for them. I would hardly call it a waste of effort because of that.
I'm afraid of wasting my time learning totally new interactions that are less useful and get changed again in 1-4 years. I would rather cling to my 'legacy' interactions that keep working for as long as I can, than to keep churning.
I was never a KDE person, but GNOME 2 was getting close to having everything right before GNOME 3 won the race to ruin desktops.
They could have continued to extend the desktop with these features, but chose to reevaluate the desktop instead. Part of the reevaluation was GTK 3, and part of it was CSS for Shell, I think. I agree it could have been better. I don't agree that the rich choice between DEs (and richness within each) is worse today than it was 15 years ago.
I'm fairly certain these were easily available as plugins on Gnome 2 with Compiz.
There was a patch for a global menu but IIRC the Gtk developers blocked that feature so it never went anywhere. Does Gnome support a global menu nowadays?
Window snapping was supported as well - many window managers support that for years.
Also note that Gnome 2 allowed you to use any window manager you wanted, so even if Metacity (the default one) didn't do the trick for you, you could use some other window manager (and it wasn't uncommon for people to do that).
Also, back in the early days of KDE and GNOME you had applications using all kind of toolkits. A DE providing applications with one sole toolkit (be it Qt or Gtk) was a novel direction in the Linux world.
The long and the short of it is, this is why incest is bad.