I am not sure if Digia walks away from Qt, there will be a lot of full time developers left. Being managed by a non-profit doesnt preclude commercial participation in any way. It merely provides a level playing field and can very well *increase* commercial participation. Ex: Eclipse.
The claim of one defacto toolkit is bogus and you are very much overselling it considering the presence of desktop environments like GNOME and Xfce not to mention a fairly large number of third party and ISV applications. Ex: VMWare, Adobe etc.
Toolkits in Linux is not even limited to merely Gtk and Qt either. Firefox and Libreoffice for instance have their own toolkits but imitate GTK look and feel. There are a odd few apps using WxWidgets, Fltk and so on. This comes at a very significant cost. Note when Libreoffice finally got font anti-aliasing.