The point, as I tried to make clear, is that those 2^32 nonlinearly encoded values are spread over a much larger linear range. If the values in the linear range that are skipped over are not discernable due to noise in the system, it is just as well they were left out. As I pointed out in my initial post, Nikon's "lossy" compression works to the extent that it is properly implemented because of this fact.
So what? What you're advocating here is a form of lossy compression, a gain achieved by quantizing multiple large integer values to the same floating-point value. In practice, there is no difference between doing that and using standard RGB integer values with a defined gamma/TRC curve. It's nothing more than an alternate method of gamma encoding. As such, it is just as undesirable to use JPEG XR as a RAW format as any other integer-based RGB file format with a gamma >1.
Let's go back to the question posed in the topic title--can JPEG XR eliminate the need for RAW conversion? Definitely not. Before the linear RAW data from the sensor can be presented as an image, a bunch of stuff needs to be done to that data. At a minimum, we must:
1. Demosaic the RAW data to linear RGB.
2. White balance (scale the demosaiced RGB channel values so that neutral colors have equal R, G, and B values).
3. Convert to an RGB editing space (which involves changing the TRC and using some sort of camera profile to convert from demosaiced RGB values to the appropriate destination space RGB values).
In-camera processing rarely does any of these tasks optimally. The best demosaic algorithms need more computing horsepower than is typically stuffed inside a camera. Shooting conditions can easily fool automated white balance sensors, and often the preferred white balance chosen for creative reasons rather than absolute technical accuracy. And given camera-to-camera variation, being limited to the standard factory color profile baked into the camera firmware is usually far from ideal. On top of that, there's the issue of level and curve adjustments that are best done on an image-by-image basis--no camera firmware is good at doing custom curve adjustments.
RAW/DNG is the closest thing you can get to the unadulterated original data straight off the sensor. Current lossless compression algorithms can shrink standard RAW file size to something fairly comparable to JPEG XR, but with no adulteration of the RAW data. The whole point of shooting RAW is to have the unadulterated RAW data to work with, so that you don't lose any DR or color gamut from in-camera processing, and have maximum flexibility to edit and adjust the image without causing unacceptably high levels of artifacts. JPEG XR doesn't change that one bit.