Both Wireshark and VLC successfully migrated to it, though I guess they have somewhat simpler GUIs.
Not really. Things are not all equally good. I don't see any effort from Wireshark or VLC to move back to GTK.
> And rewriting to Qt would be as big as starting new projcet from scratch
Well that's clearly an exaggeration but I agree it would be a huge project. I'm just dreaming...