The first question I'd ask is what Microsoft have in place for a successor. It may not be logistically possible for them to ramp up and have something available for 2016 or even 2017 right now.
Let's say they do have the ability the question then is how much of a jump in power are we talking, the price point, and the library it would have. Making a half step seems foolish, but so does making something truly "next gen" that cost $600-700 range. Would they shift over X1 games abandoning the system, make game cross platform, or have droughts to start?
I don't think the Xbox brand has the global influence to make such a power move and expect the industry to move with it. I also doubt Microsoft as a company would be thrilled spending on the research and development for a new system that almost always sees consoles companies dip into the red for a while.
