Who says? FWIW I don't believe that the flat spot has anything to do with the sensor at all -- it is due to the noise being dominated by that of the ADC which follows the sensor in the Canon designs. If you really dig into things you will find that the Canon sensors are actually the class leaders in terms of dynamic range, they just foul it up in terms of their system architecture. The answer to the question "can Canon deliver a class leading sensor" is yes... they already do. Take a look at the data from Sensorgen for the 5DIII for example: Min read noise 2.4e-, FWC=67531 for a DR of 14.7 stops.
And Nikon's D4 would whip everything from Canon quite nicely then.
If they were to put an ADC on-chip similar to what Sony does, they would have equivalent performance maybe even better.
What Canon needs is a "class leading" system architecture and ADC. This is a nit, I know but it is important because Canon DOES have class leading sensors and has had them for quite some time.
If you mean "system architecture" to mean "sensor architecture", then yes because the ADC is an integral part of the sensor.
When I say system architecture, I mean exactly that -- how the system is architected. In Sony's case they have optimized the architecture to minimize pattern noise and overall system read noise -- it is an excellent design in that respect. They achieved this through the use of a distributed ADC structure that they were able to implement on-chip. It is a clever and innovative approach (originally invented by IBM, BTW).
Canon has chosen to design their system in am more traditional way using an ADC off-chip. This has consequences, since it requires a higher speed, more complex pipelined ADC which will not yield the same effective number of bits (ENOB) of the Sony approach.
This is a classic study in system architecture where you can design two radio receivers using the same high performance front end LNA (low noise amplifier) but without optimizing your noise lineup for DR one doesn't work as well as the other. In this case it isn't a satellite receiver but a camera but the principles are the same.
FWIW, the ADC that canon is using is state of the art, they have a 14 bit converter that is probably getting about 12.5 ENOB at 40 MSPS. That is about as good as it gets for a pipelined ADC running that fast. Both companies have state of the art sensors which appear to yield pretty much the same performance, Sony has done a better job on system design by optimizing the system noise line up for this one particular parameter.
If you are going to talk about sensors you need to talk about sensors, the sensor is only one part of the overall system
and it is this overall system implementation that determines the noise figure and dynamic range of the camera. In the case of Canon the sensor and ADC are separate chips.