Big megapixel is a "medium format killer", just as D800 currently is. It's targeting a specific genre, high resolution photography. It's not a journalist camera like the flagships. Sure, for typical 35mm photography high resolution is just a waste of disk space. But a high resolution 35mm is there to stretch into medium format territory, just as medium format digital stretched into large format film (think 4x5 and 8x10 view camera) territory. In other words expand what you can do with a 35mm system. A pro Canon shooter could have a 1DX for fast handheld action work, and a big megapixel camera for studio/still life/architecture/landscape.
Probably the high resolution genre is smaller, but every landscape hobbyist will want it (those are many!), and some of the pro shooters that use medium format today will drop the costly MF system and use only 35mm for convenience. I think Canon need this type of camera in their lineup in the long-term to provide a cameras for all genres users nowadays expect 35mm to be good at.
In the medium format forums the only camera that is considered as real competition with MF is the D800, and indeed several has ditched MF in favour of the more user-friendly, all-around and cheaper D800. In the same forums Canon is still used as an example to show off how "bad" 35mm is compared to MF, as it still has poor dynamic range and color rendition at base ISO compared to MF, while the D800 actually is competitive and even better in some aspects.
I am not sure I'd call 30-50mp DSLR's "medium format killers". I would probably term it "medium format intruders". When the most important thing for your work is pixel count, there is no denying that medium format has the edge. Medium format sensors have both higher megapixel counts (80mp, or even more with advanced hardware like Hassy's 200mp multishot), as well as larger pixels than any relatively comparable FF DSLR (i.e. the pixel on a 40mp MFD are going to be meaningfully larger than a 40mp FF DSLR). Granted, I think that the per-pixel technology and per-pixel quality is probably a bit higher on DSLR sensors...CMOS sensor technology has been pushed very hard, very far, very fast over the last few years, while the primary innovation in the MFD sector has been...yup, more pixels. I definitely think FF DSLRs are closing the gap, and are indeed intruding on MFD territory...but they aren't far along enough yet to call them medium format killers (and they will probably never really reach that point...in the areas where MFD rule, the only thing that really matters is raw pixel count...and with more surface area per sensor, you can always pack in more pixels than you'll ever get on FF DSLR, even if the DSLR pixels have higher per-pixel IQ.)Not a single recent sensor out from Canon is even remotely close Sony Exmor sensors in terms of base ISO performance. I'm still waiting to see that Canon actually can produce a sensor which has the properties high resolution photographers desire - ie great dynamic range and great color fidelity at base ISO. High ISO performance (which Canon indeed is good at!) is not irrelevant, but much less important than in traditional 35mm photography.
Canon's problem here is high frequency off-die components (i.e. ADC). With a die shrink (not confirmed by any means, but I think likely, if not even necessary to achieve higher pixel densities), Canon could drop a lot of their off-die components right onto the CMOS sensor die, go column-parallel, and get considerable improvements at low ISO. It should be noted that Canon's per-pixel CDS performs better than Sony Exmor D-CDS at higher ISO...read noise on an Exmor doesn't go below ~2.6e-, where as on a Canon it drops to as low as 1.5e-, which is part of the reason Canon high ISO output is so clean.
I would also dispute the notion that high ISO is less important in digital than in film. There are far more photographers who use higher ISO settings these days than those who use only ISO 100. Sports, wedding, journalists/paparazzi, studio (not ultra high, but frequently enough higher than ISO 200), wildlife, birds, airshows, and pretty much anything with action. Even in good light, when you need to freeze action, it is not very hard to fin yourself at at least ISO 800. Most of the wildlife and bird photographers I follow are increasingly becoming comfortable with ISO settings as high as 10k, and in some cases I've seen a couple professional quality, printable photos taken at ISO 16k taken with a 1D X!! Don't forget night photography, street photography, night sky/deep sky astrophotography (which is usually between ISO 800 and 3200), etc.
The types of photography where ISO 100 is king is far lower. Landscape photographers are probably one of the most obvious groups, studio photographers are also a group that can make good use of low ISO. Still life photography, product photography...probably anything static where you are manually, directly lighting your subject is likely to use ISO 100 and 200. Still, add up all the photographers in the world who do this kind of photography, and you'll still fall far short of sports photographers alone, let alone the entire community of photographers who rarely use an ISO setting below 400. The MFD market is small because it can be small...not as many people need both superior ISO 100 performance AND unparalleled pixel counts. For every studio photogapher with a couple Hassy H4D-50's, there are 50 sports photographers.
Well, Nikon has cooperation with several manufacturers regarding sensors,Nikon/ Toshiba has 1.4 e read out noise and maximum efficiency of all sensors today. Better than Nikon/Sony, and then you have Nikon/Renesas with 1,9 e read noise
Being alone in in the sensor development /manufacturing is no longer a strength that it was in 2004