I know I’m part of the minority in liking the Gnome 3+ designs, but with so many people lamenting the death of GTK+2, why don’t they fork the toolkit? It’s not as if you’ll break any compatibility by backporting fixes and extending the classic UI components.
Perhaps you’ll need to rename your project (except for the system libraries) to avoid trademark issues, but if all the developers came together, I’m sure you could write a drop-in replacement for the old GTK+2 libraries. Such a project may have some difficult tasks ahead of it (bringing Wayland support and fractional scaling, for example) but they can copy Gnome’s homework, they don’t need to invent everything from scratch.