I think that MS's success is that they have the timeline first, then figure out what to fit into it.

The problem comes when the functionality and the delivery date are announced at the same time. This violates a law of nature: the more you know the delivery date, the less you know what will be delivered, and the more you know what is to be delivered, the less you know when it will be delivered.

This urge to violate said law of nature comes from a very reasonable place: the desire to know how long and how much a project will cost. Ultimately, however, I think that many if not most failed projects come from that place.