Well, motion sensing is HD since it tracks eye brows, for my standard. As for video conferencing, since there is no need for depth, just video conferencing, you can turn off depth sensor and give 2D camera more bandwidth.
I understand what you mean, but, it is not a must. USB3 will be used for sure. And when that happens, certainly you can use both 2D and depth in full resolution.
But, changing the kinect hardware itself would be a major problem for marketing and user "trust". Without clear demand on HD version of kinect, people would not want to replace the old one, but, will be angry that they HAVE to upgrade when you put that out in the market. Even if it is optional, they still feel obligated to get the new one. You don't want them to think what they bought is junk in 2 years, unless you are Apple.
But, just to make you feel better. I do believe they will release a new kinect two/three years after Xbox Next. The bottleneck would be expect and they would want to refresh their hardware. And that also don't require customer's money all at once at console launch.
In short, USB3 for sure since it is cheap. Kinect HD is expected to be later as Xbox Next Refresh. Just my speculation. I could be wrong. I don't work for MS.