A nice story with a bow on top, but it doesn't match reality. Flat design started at Google and spread to Microsoft and eventually Apple. It was simply the style, and now the trends are starting to change. We'll go through a 3D glossy "aqua" phase for the next 10 years, then we'll go back to brutalist flat design. And we'll have no choice but to follow the UI designers into whatever obsession they're on time after time.