Lately I've been thinking a lot about the subject. It's no news that GNOME 2/GTK+ 2 crowns the perfect balance between configurability and elegance, but by this point it's crystal clear that its ideals are no longer in line with the current GNOME project and it's only going to get worse with GTK 4 as pointed out earlier.
I'm almost sure that we need a new toolkit, whereas it's a fork of GTK+ 2 or a reimplementation that draws from its concepts.