That fork of GNOME with a Win9x-style UI exists: it's called Cinnamon.
It's OK and it's quite popular. It's a lot more customisable and versatile than GNOME's own GNOME-Classic or GNOME-Flashback sessions, but it's still really quite clunky compared to, say, MATE or Xfce.
If GNOME-Classic or GNOME-Flashback worked a little better, so that one could rearrange panels and add/move/remove controls and so on, I suspect that there would be no need for MATE to exist.
I personally find Xfce to support more of the customisations I personally want (such as a vertical taskbar with horizontal rows of components and controls) than MATE, or KDE come to that.
Wayland, for me, delivers nothing I really want. I read about variable refresh rates, HiDPI support, smooth flicker-free animations and so on, but I don't actually want or need any of those things. I don't get flicker or tearing as it is, and X.11 does all I need: seamless multihead support with 3D acceleration, and so on. I frankly prefer a minimum of animations and fades and so on.
Forking something and keeping it in a past state isnt really as useful as some think. See OpenOffice vs LibreOffice for example.
This is actually OT because this is about GTK, which is a fine toolkit even if it's not C++. It has binding for most languages.