I think this is a bit misleading. I those numbers are based on linear pixel density divided by FOV (for the DK2, 1080 pixels/100 degrees). That is OK to a first approximation, but the LEEP optics in the rift do not evenly map the pixels. The pixels near the center of the screen are much less stretched than the pixels at the edges. This is appropriate because our eyes have better resolution near the center of our retina. If you are principally looking forward as you are when using your real monitor, the effective pixel density of the HMD is going to be much higher than stated above. If you are looking out of the corner of your eye, it will be much worse. Assuming HMD continue to use flat panels with LEEP optics, a proper 4k panel may be adequate to allow proper desktop representations. Of course, all of this math changes once we start using curved OLEDs, etc.
There is also probably some subpixel rendering improvements that can be done as well. I continue to be amazed at how much readability improves when using ClearType or similar subpixel font rendering even on high DPI monitors. Of course, the same subpixel ideas/antialiasing ideas may need to be applied to the entire windowing system allowing LEEP distortion/viewing angle compensation for borders, widgets, etc. There are lots of opportunities here to design a 3D windowing interface and get all of these things right. I'd love to have the 27" and 30" monitors on my desk to be the last I ever buy.