Your theory assumes that a D4x will increase resolution while maintaining image quality, like colour rendition, shadow noise (low ISO), high ISO noise, dynamic range. I doubt this will be so easy.
I believe that Canon and Sony will also achieve similar things, this is not just about Nikon.
As far as Nikon goes, between the D3 and D3x they managed to double the resolution while adding more than one stop of DR and improvig the pixel per sharpness. So they have done it in the past. I don't care about high ISO much, but it will still be for sure superior to MF performance.
Let's compare the progress between 34mm and MF:
- Jan 2006
- Nikon D2x: 12 MP, 11 stop DR
- P45: 39 MP, 12.5 stop DR
- Jan 2008:
- Nikon D3: 12 MP, 12 stop DR
- P45+: 39MP, 12.5 stop DR
- Jan 2010
- Nikon D3x: 24 MP, 13 stop DR
- P65+: 60MP, 13.5 stop DR
- Leica S2: 37 MP, 13 stop DR
I also agree with the other answer that a - let's say - 32MP FF sensor will be increasingly diffraction limited. Or you will have to shoot at F5,6 (which limits DOF considerably, and therefore resolution) or you will get files with larger DOF but softer/ with less sharpness.
There is a 1.6 surface ratio between 24x36 and 30x45, which means that the pixel pitch will be the same between a 33 MP d4x and a 50MP S3, correct?
Both lenses will need to be stopped down to the same aperture to avoid diffraction, but you will get more DoF with the D3x than with the S3 at equal framing since you will be able to use a shorter lens.
You can also reason at equal resolution and will reach the conclusion that there is in fact no difference as long as you keep using lenses that are good enough between f4 and f8, which is the case for most modern design primes. It is true that there will be a point where the DSLR will run into a wall, but if 35 MP is not enough for you, then 50 will also not be enough, you will need to stitch anyway.
I know a photographer who has used Mamiya ZD, now SInar MF back, Leica M9 and D3x. He says, quality wise the MF digital comes first, M9 second, D3x third. Obviously he means esthetics, colour and the final impact the photos make, not speed and high ISO performance.
So did I... and my assessment is very different. I see the D3x files are being superior to the ZD files in every single domain, most obviously shadow noise.