And adopting and modifying GTK-3 seems like a battered woman going back to her abusive husband... The FSF has a sorry track record of destroying everything they touch, and the GPLv3 getting forced on all their software is making individuals and companies run for the hills at a record pace.
You can't blame FSF for what happened to GTK3. Besides, GTK3 is licensed under LGPL 2.
No, if you want to point fingers at GTK3, blame the developers. Also note that GTK's developers do all their work from the command line -- not exactly the people you want designing a GUI toolkit.