Although this is my complete guess, I think XB1 is going to hit rock bottom this January. XB1 were relying on the "best lineup in history" to give them some momentum into 2016 but it just hasn't happened. MS are going to need to keep that $300 price tag to keep good sales through the "off season" and compete with a $350 PS4. I think this is why they want to try for the "best lineup in history" again in 2016 and and keep people interested.
It might look rosy for MS right now in the US but I'm not sure where they can go from here.