Why do you hope they abandon GTK?
What would you have them replace it with?
What would you have them replace it with?
They both have kinda similar roots in that XFCE originally used XForms which was an open source replacement of the SGI Forms library while FLTK also started as a somewhat compatible/inspired opensource replacement of SGI Forms in C++.
If they ever move away from GTK (due to the GNOME shenanigans GNOME-izing GTK) I wish Englightenment and Xfce were together a single thing. But that's if I could ask the Tux genie three wishes.
But frankly I think forking and maintaining GTK3 is preferable to moving to EFL or Qt. GIMP is still on GTK3. MATE is still on GTK3. Inkscape is still on GTK3 (but GTK4 work is in progress). Evolution is still on GTK3.
I think GTK3 will be around for a long time.