I've run out of time for big swathes, sorry, but I had to comment on this. 'simple' is not the ideal goal of UI design. 'appropriate' is. You want a UI that is best appropriate for the situation. Sometimes this will indeed be simple. Other times it will not be. As an extreme example, could you imagine a cockpit with a single hardware button and a small grid of soft dials?
A more moderate example is that to do serious business work, people still need desktop environments - tablets do not cut it. Mobile OSs are too simple for most serious business work - they don't have an appropriate UI.