It is not only about the sensor.
Lenses. There are those that say that we are no-way near outresolving lenses and those that say we're already way past it.
I agree that lenses are likely to be the ultimate limit on useful sensor resolution in a given format. But the evidence (like that you describe) is clear so far that the D800 gives significant improvements in resolution over 35mm format sensors of 24MP or less from a wide variety of lenses, even though the extent of the improvement is less towards the corners, less with some lenses that others, and less at the extremes of the aperture range (due to diffraction at small apertures, lens aberrations at large ones).
I am mystified by the idea that there is a "natural limit" on sensor resolution for a given format at a point (22-24MP?) beyond which clear further gains in resolution are possible, simply because the extent of the gains varies with the circumstances.
After all, differences in lens resolution were clearly visible with film, and even with films of less that the highest resolution. Especially when one examines the corners of the larger prints encouraged by higher resolution films. Yet no one suggested that new higher resolution, finer grained films went past some natural resolution limit (of, say 83 lp/mm, which is the absolute upper limit for a 24MP sensor in 35mm format.)
I will repeat that the D800(E) does not even match the resolution of some monochrome films. For example, TMAX 100 and 400 have distinctly higher resolution and better MTF than the D800 or D800E, and I have never heard of anyone criticize those films as going beyond some natural resolution limit imposed by lenses.
- a 24MP sensor in 35mm format has pixel pitch of 6 microns, so an absolute upper resolution limit of 1000/12 = 83 lp/mm
- a 36MP sensor in 35mm format like the D800's has upper resolution limit of just under 100 lp/mm
- TMAX 100 has MTF of 70% or better all the way to 100 lp/mm, and MTF 50% or better up to 125 lp/mm
- TMAX 400 has MTF of 50% or better to beyond 100 lp/mm
And in reality, Bayer CFA demosaicing reduces the resolution of those sensors below those maximum (Nyquist) values.
It will be time to complain about "excessive sensor resolution" when sensors surpass the resolution of _all_ the films that have been popular in 35mm format for the sake of their high resolution.