There is all the saying about how much ahead Sony sensor is vs Canon and that the 5D3's sensor has not improved from the 5D2.
Firstly, the 5D3 sensor uses a gapless microlens design which was not used before. Numerous test have shown that it has improved over the 5D2 by at least 1-2 stops. So it is quite unreasonable to say that the sensor has not improved.
Secondly, I noticed that most of the test showing the D800 having a better noise performance than the 5D3 often involved downscaling or upscaling the image to the same size... When showed at exactly the same size, it is often obvious that the D800 is noisier than the 5D3. When downscaling is involved, even the Nokia Pureview 808 showed remarkable improved noise performance. Now, is this purely due to sensor technology?
Given Canon's 18mpx APS-C sensor technology, they can produce a 46mpx FF sensor without having to do too much. At the noise level of the 60D and if one were to downscale the 46mpx image to 36mpx (or 22mpx), I am very sure the noise level will be very comparable to the D800 or better. Shadow noise is a real concern for Canon however. Hopefully they can start putting in an on-chip ADC soon.
Thirdly, DR. Perhaps someone can show me proper photos showing what those 3-4 EV advantage that Sony sensor supposedly boasts as compared to a photo taken with a Canon camera. I would be convinced then. Right now, all I hear is 'scientific' tests with a bunch of numbers. A proper set of photos would convince me what advantage a Sony's sensor has in terms of DR. So far, all I have seen is a bunch of numbers.
Lastly, the whole point of all this megapixel. In DPReview's tests, they admitted that getting that observable resolution advantage from the D800/D800E required a lot of care and effort. Only in lab condition and at very specific lens type and setting can they achieve that resolution advantage. All this while, one has to carry around that extra bits of file size and run out of drive space consistently.
If we look at the AF speed comparison on imaging-resource.com, it is very obvious that the 5D3 focuses almost 2x as fast as the D800 and do not suffer as badly from delays in buffer clearing, etc. That in itself makes the 5D3 a much more usable all-rounded camera.
So Canon should make a high mpx camera, by all means. Landscape and studio photographers would love that. It would definitely not affect the target market of 5D3 - People who need an all-rounded camera.