Hi Victor,
Just a small point, diffraction will not really limit small pixel sensors if used at moderate apertures like f/5.6 or f/4. Just as an example the A7rII I have produces significant moiré at f/8, so I guess that it would need around f/11 for diffraction to limit resolution.
From that I guess that optimum pixel size woould be somewhere around 2,5 microns, or say 140 MP.
DoF at 140 MP and f/5.6 would of course be rasor thin. With a larger format you would need to stop down a bit more and you would run into the very same diffraction limit.
Another way to see it is that diffraction only depends on aperture diameter, that decides the angular resolution. With a longer lens the same angular resolution will be projected larger. That is the reason that astronomers are that interested in telescope diameters.
I can post an example later.
A guy called Jim Kasson has done a very detailed analysis and the image below illustrates some interesting things:
What it shows that MTF 50 is pretty constant with decreasing pixel size at f/12, but even at f/8 there is a significant benefit of reduced pixel size.
Jim's posting is "kitchen physics" I think he would say but still takes some effort to read:
MTF50 vs f-stop and pixel pitch
Best regards
Erik
Diffraction will put unbelievable constraints on any 60 or 70MP 35mm size chip.
Victor