I believe Sony Digital Imaging is separate devision to the Sony Sensor Business, this means they are not allowed to withhold sensor technology availability to direct competitors, it would also mean Sony sells less sensors which means less profits. Sony DI do however get a 6 month exclusivity on newly developed sensors, this is why Sony DI has been churning out new bodies quickly to utilize that 6 month advantage.
I believe its down to competition law(s), however the cost per unit is determined by the Sony Sensor Business so it could be they artificially price the Sensor at a level where most of the competition is put off.
I would guess the new Nikon D820/850 will get the 42.2mp sensor which the Sony A7RII has used.