I don't think Canon planned to release the 5D MkIII as it exists today. I think Canon intended to put a much higher MP, lower noise sensor with greater DR in it, but it wasn't ready in time. This is all pure conjecture.
I think Canon was fully aware of the Sony sensor and Nikon's plans, and physics and electronics being what they are, was able to produce a sensor equal to the one in the D800 using Canon technology. There's nothing revolutionary about the Sony/D800 sensor. But I don't think Canon could get it produced in the quantities they needed.
I suspect there was a quality control/yield issue. Yes, the sensor worked, but not enough of them were coming off the production line that met their standards - too many rejects in each batch.
This left Cannon with some choices:
- Release no update to the 5D MkII, already 3+ years old until the sensor was ready, and in so doing, appear to cede that market segment to Nikon.
- Put the top of the line 1D series sensor in the MkIII - an improvement, but the only way to maintain the price point would be to take a loss/break even on every body.
- Put a slightly tweaked MkII sensor back into the MkIII along with the other planned feature upgrades and basically mark time until the new sensor was ready.
I think they took the last option - issuing a minor upgrade so as not to be seen abandoning the market segment to Nikon.
Nikon took advantage of the situation by knocking down the price, and I think that, more than anything else caught Canon off guard. Canon would do well to take a price cut on the MkIII.
Why did Sony and Nikon succeed with the new sensor where Canon did not?
I'm not so sure they did. First, I think Nikon may be creaming the sensor production - paying a premium to Sony for the very best of each run. They don't need that many, since the D800 is a relatively expensive, low volume camera.
A company Sony's size may be able to afford production runs 100x or 1000x larger than Canon. Their yield of the very best sensors may be no better than Canon, but they may have buyers for the lesser sensors - other camera makers and other Sony models that may be happy with fewer pixels, more noise, lower DR or even, via trimming, smaller sensors.
Or Sony can afford to take a loss on initial production runs, knowing everything will work out in the near future. Sony did this with the PS 1 and PS 2 gaming processors.
It could also be that Canon's sensor production facilities for this model were in Thailand, were flooded out, and the replacement factories are not fully on line yet or tweaked for higher yields.
I think Canon's next full frame camera will be very informative. If it is in the 18-24 MP range, but retails near the 7D, then I would bet that these are based on the same sensor that was originally planned for the 5DMkIII, but represent the lesser quality production yields. (18-24 usable low noise, high DR pixels vs 36-45)
I bet that within a year, Canon updates the 5D MkIII type camera with a much higher MP count, lower noise, higher DR sensor. They may not call it the 5D, but it will be what they had originally planned for the MkIII.