I don't sense the fear people are having when it comes to an iterative Xbox 1.5. If it can indeed share a library with the original, but has settings for better visuals and higher framerate and resolution if a developer allows it then it seems like a good plan. I'm sure Microsoft wouldn't make it difficult and I would not be surprised if the SDK accounted for it a while ago and the changes could be automated.
Once UWP becomes a larger and a better platform, having games that perform better on the X1.5 could also be a selling point. Microsoft could add the Oculus Rift store and allow Rift connectivity so they could say they support VR without as heavy an investment as Sony.
Not sure what else they could add aside from the 4K uprez and multimonitor/external display support for multiple users on certain titles and functions, but that has too many limitations.
While that personally wouldn't be enough for me to want to update, it should be enough to keep the platform current and adaptable to future technologies, which is their goal. Keep the current offerings, perhaps miniaturized, as the lower cost entry-level offering and set the new X1.5 as the premium offering.