I agree that the XOne isn't doing as well as its current sales comparisons with X360 may indicate. That said, I also don't believe that Microsoft is going to be in a rush to replace the XOne (if at all).
The X360 (like the PS3) was initially heavily eclipsed by the success of the Wii. The SD world was still very much alive in the years 2005-2008 and so the HD consoles of last gen were especially back-loaded when it came to sales (they were deliberately built to be ahead of their time). Selling on par for the first 3 years of the X360's life does not necessarily mean that the XOne will be able to ascend in sales the way the 360 did in 2009 and onward. In fact, in my opinion, the XOne has already thrown a lot of its best punches with aggressive price-cuts and two major Halo releases. To make matters even more challenging, the NX is most likely coming out this year and if rumors are correct the home console component will likely run on par with the PS4. As a result, starting this Christmas season, Microsoft may have the weakest of the three consoles and this is a position that they have never been in before. I'm not saying that the XOne will end up being a failture, but I don't think that it's going to sell anywhere near that of the 360 and I even think that the 50 million milestone is going to be challenging for it at this point.
That said, I do not believe that Microsoft will rush another console out. Unlike Nintendo, Microsoft doesn't rely on gaming for its survival and even their gaming division doesn't rely on console gaming exclusively. There are increasingly other options becoming available and I think Microsoft is going to be heavily tempted to move away from the traditional dedicated home console market after XOne. Year after year, Sony is doing the exact the same thing as Microsoft but with far more success and if Nintendo comes back with a vengeance and starts to crowd the market even a bit more with the NX then I think Microsoft is the company that is most likely to duck out. Yes, Microsoft has the resources to fight the console wars with the other two companies but they also have the least skin in this battle. If they see opportunities with an online service as their next platform, I think they will be very likely to take it.
