The sensors I tested were of the same generation and of about the same QE.
That's not true at all. The G12 has an extremely advanced sensor that has a QE of 55% the 5D has a QE of 25%. That's more than a stop of difference. You're comparing stone age technology to the space shuttle.
I think the problem with your test is that you have to account for the number of t/stops in each camera's respective lens. If you actually shot the images above at the same f/stop, shutter speed and iso and didn't just try to get the exposure the same, then most likley the lens you were using transmitted twice as much light onto your 5D as the lens on the G12 due to the quality of the glass.
In any case. The point is that while it is possible to make a 300 megapixel monster of a camera with 55% quantum efficiency, they haven't done so. My guess is that the cost of scaling up the G12 sensor makes it not cost effective.
I'm positive that Canon has the technology to make a camera that has 36 mp and performs better than the D3s in low light and dynamic range. Unfortunately what is most likley going on is that they realize that such a camera would end up costing more more than creating 2 seperate bodies, one for resolution and one for low light.
It actually doesn't matter whether Canon has that kind of technology or not, nobody denies that Canon has excellent R&D in this industry.
It just doesn't mean anything to end-users if Canon don't put the technology in their products. Someone mentioned Canon is a conservative company which I totally agree.
Canon can still maintain their sales figure ahead anybody else just because there's no company like Apple in this industry.
IMO, Nikon focus on how to make better DSLRs and Canon focus on how to sell more DSLRs .