The only problem with this argument is that Windows and Office are both more popular than they've ever been and industry-wide, people are purchasing more desktop/laptop/notebook PCs per year than ever before. So if by "dead" you mean "more popular than ever" then I guess this argument holds water.

http://techcrunch.com/2010/06/26/microsoft-numbers/

It's true that of late the focus has been mobile, mobile, mobile, and Microsoft missed the boat here to a degree, but only in comparison to what Apple did, which is start a revolution around the idea of easy, mobile devices. This is a completely different market than the desktop market, and the popularity of one doesn't mean the death of the other. By the way, people have been calling for the death of the desktop since the 80s. 90s. 00s. Hasn't happened yet. Down the road...who knows.