Going to the extremes of a 400MP FF sensor will allow you to see 400MP of beautiful blur up close when viewed at 100%, unless a lens exists to resolve 400MP of resolution at f2.3 and wider apertures.
I recall seeing on another forum that someone had tested the resolving power of a Canon 24TSEII on a Pentax Q with the rationale being that 12.4mp on the tiny sensor would correspond to a 360MP full frame sensor. The uptake of this experiment was that the 24TSEII obviously delivers the necessary resolving power. So I don't think it's that big of a stretch.
Granted, 300-400MP may not be practical for many photographers, but it certainly is possible with lenses that exist today. And we are likely to see many more lenses with improved resolving power in the future.
If that's the case, my calculations are out. I was basing it on DLA as quoted on the-digital-picture, and found a website to allow you to show what various cameras pixels sizes are, compared to the Airy Diameter of a perfect lens at a given aperture: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
Anyway, to calculate the pixel size of a sensor in µm, it turns out the calculation is =36/(sqrt(MP/2*3), and the ratio between DLA and pixel size seems to be 1.6 (to match up with Bryan's calculations at TDP). So my calculation was DLA = 57.6/(sqrt(MP/2*3). Which is according to your above post about the Pentax Q1 and 24 TS-E II, is incorrect.
If that is the case, 39.3MP is just a stepping stone towards much higher resolutions for those that demand it. The next logical resolution for video will be 39.3MP x 4 to allow for perfect downsampling of 1080p, 2k and 4k - and that's 157.3MP if the sensor is scaled up to 3:2. And stills? Who knows? I had assumed we were closing in on the practical limits as governed by diffraction with this next generation of sensors, with a possible sensible upper limit of around 70MP.
However, the Nikon 70-200/2.8 with a Nikon V1 seeing the centre crop at an equivalent of just over 70MP does fit in with my calculations - the lens at f5 is stopped down sufficiently to get close to its sweet spot in terms of sharpness, but crucially not beyond f5.6 to get diffraction setting it. However, that a FF lens such as the Nikon 70-200 is good enough to resolve that much detail does surprise me. It would be interesting to know from anyone using a FF lens on a V1 if the results suffer when moving too far away from f5.6?
It would be interesting to see an f3.5 lens resolving, even in the centre of the frame, enough to look truly sharp at 360MP when viewed at 100%. According to my calculations (which may be out), 360MP would need f2.5 to avoid diffraction, and a perfect f3.5 lens will top out at 180MP of resolution.