I'm torn to think if Sony will upend itself on the 50mp products. Why would Sony sell a 50mp 24x36 sensor when it makes a killing on the 50mp 33x44 sensor? If anything, I would expect the next gen 33x44 to launch & be active in all forms before 50mp comes to 24x36.
The 42mp 24x36 is a nice compromise, they're not lagging the 50mp Canon in any way other than dots, and they have a lot of other things they could put into a body that would draw more attention & sales.
They make a lot more profit selling cameras than just sensors.
And many buyers of cameras featuring the 33x44 sensor are attracted to it due to the mythical qualities of MF.
Having a 50+ 35mm sensors will not have a significant negative impact on the sales of those small MF sensors. Besides this is already old story for Sony semi-conductors, they already sold a majority of those 33x44mm, including those to be used in the majority of the X1D and GFX bodies that will sell. They are now focused on the second versions of those cameras with the 100mp 33x44mm chip.
So all in all, I am 100% sure that a higher res a9r is coming with anywhere btw 50 and 70mp.
Another way to look at it is that the availability of an excellent 20mp 1 inch sensor in the RX100 series has never been seen by Sony as risking to endanger the sales of their best selling 24mp APS-C sensor, has it?
If the rumors of them cutting out Nikon from best in class sensors are true, they also have no choice but to move forward in resolutuion because Nikon won't be standing still at 36mp. Canon will also keep moving to. The current 50mp they are trying to push in the 5DRs is old tech even by Canon standard, but they can't admit that, so the 5DRs II will have to be at least 70mp and should be coming in 2018.
Sony has no choice really.
Cheers,
Bernard