Why would they? It would cost billions, take years and mostly consist of dead-end features which Windows keeps around for compatibility. The end result would be

Why would they? It would cost billions, take years and mostly consist of dead-end features which Windows keeps around for compatibility. The end result would be an almost worthless product which always trails behind Windows and which MS can dominate with their existing software. It would also massively increase the market for "real" MS Windows. Don't extend your competitors platform. Chrome took IE users and made them Chrome users because Google's platform is the web, not the desktop. So Chrome has been pushing forward traditional desktop features such as offline, OpenGL, and sockets, to displace desktop software. Look at what Apple has done with the iPad, they've been the market leader and MS has had to follow them, only recently shipping Office apps for iOS. They're blazing ahead with their own platform which serves their goals, not tired old legacy software from 30 years ago.