I agree that the D3/D4 sensors are custom built and perform very well. However, will a Nikon D4 user be satisfied when a new D4 would come with only a 16Mp version. To make clear, I would have no problem with that as I also have a 1Dx with 'only' 18Mp. it's not only the pixels that is counting. It's the complete system. Would the same people say over here then that the D4s is 2 generations behind on the D810 with Sony's sensor?
They have access to a fab that can handle 35mm sensors and at least two companies who create smaller sensors that perform on par with Sony's. Working something out shouldn't be impossible. It's hard to say no to the company that makes the machines you need to make your semiconductors after all
Not the most pleasant way, but strategic deterrence involves options that neither party wants to become reality.
The two generations isn't about the resolution, but about signal quality. Its much easier to get a picture from a D4 past the editors then one from a 5D3 w/o debanding. And the debanding in turn softens the image in rather unpredictable ways. And costs money.
D8x0 otoh/additionally is more about not having to go to the Phase One, with its associated costs.
I agree fully that Nikon is multi sourcing what is a very good purchase strategic, however they are not buying the same product at all those places. The Sony Exmor with the big Mp is really top. If that source would be cutted off, they have a big problem for the Dxxx and Dxxxx series.
The Toshiba sensor is good enough to keep people from noticing that some of the current Dxxxx aren't Exmor.
As for buying Nikon - they are part of the Mitsubishi group, and those folks wouldn't want to see their supplier of lithography equipment and such getting under the control of someone else.
Thats maybe the meta-part of the story - it's not that Exmor is so extraordinary good, its just the best known, but that all but one of the others are in the same ballpark.