I think Windows 9 (or whatever it's going to be called) will be a complete pull back into the classic desktop. By the time it ships there will be Modern UI only OS hybrid built with Office for the Modern UI finally caught up. They can all have the same underlying base OS (whatever it's going to be called) with different shells on top. But, I'm not sure how programming model is going to look like, unless they allow classic desktop Windows to run apps built for Modern UI in windowed mode (which seems to be the direction they are heading towards).