Some of the sharpest lenses for medium format digital have over 65% modulation at 80lp/mm, so in practice they will have very usable detail above this frequency. With a bit of sharpening, 10-20% modulation is still valuable for digital photographic use, and I'm sure the lenses with 65% modulation at 80lp/mm could at least do 120lp/mm at something like 20%.
Additionally, oversampling is needed or you will loose detail to aliasing. Nyquist theorem states 2 times the frequency, but this is only the minimum frequency needed to be able to recreate the signal in theory, in practice you want more (try looking at a 10hz sine sampled at 20.1hz).
300lp/mm on the sensor would probably be good for really sharp lenses, on sensors with the Bayer filter removed (black and white), but only for detail along the X and Y axis. For the same detail on the diagonal you'll need over 400lp/mm along the X/Y axis.
Then add the Bayer filter, and you will need to double this again, otherwise you will loose detail, mostly on saturated red/blue subjects. So something like 800lp/mm is desirable to see what the lenses have to give. That is 1600 sensels per millimeter (two to a black/white pair). In the 54x40 sensor size used on the latest 80 megapixel backs, that would be a resolution of 86400x64000, or about 5500 megapixels.
Of course, in practice not everyone uses the best lenses (or use the apertures and precise focusing needed), and few would notice if faint diagonal detail in saturated reds was lacking, but I don't think the megapixel race is going to slow down until we reach about 1000MP medium format and 200-300MP on full frame sensors (no AA).
This is not a bad thing, in the high-end segment, pixel density increases have always gone hand in hand with dynamic range and real detail improvement. Sure, some people like to extrapolate their knowledge gained from noticing that a 14-megapixel compact might be worse than a 8 megapixel one, but for high-end low-ISO, dense is good.
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