Thank you Dustin for the thoughtful and well written review.
Oddly enough I just sold my 6D after owning the 5DsR since release. This is a reflection of what other photography gear I own, rather than a negative assessment of the 6D.
I have personally found that the 6D produced images that are a bit too soft, and I find the white balance a touch too warm. The 5DsR produces crisper, cleaner, and more contrasty images—as long as you are shooting in near ideal light. It's a studio camera after all...the "s" in 5Ds stands for studio! That is how I use mine, although as long as you don't push the ISO, it can be used out in the field too.
The reason I sold my 6D was that the latest Fuji X-Pro2, and even more so the X-T2, is starting to be able to replace it as a field camera (dual card slots, faster frame rates, increasingly better able to shoot action and in lowish light). The Fuji X-Pro2 has a white balance similar to the 5DsR. I could imagine that if you were used to the 6D that you might think these newer models from Canon and Fuji have a cooler white balance, but I suspect that these newer models have more accurate white balance compared to the 6D, which is a touch too warm. I notice this with skin tone, where both Canon and Fuji together arguably represent the industry standards when it comes to getting good skin tone, with their newer models being even better than the older ones.
The next issue is that of the advantage of high resolution sensors. It is not limited to just oversized printing at all. I tend to agree with Tony Northrup:https://www.youtube.com/watch?v=yyOmgArU0MA&feature=youtu.be
The last issue is that of the oft mentioned issue of diffraction. I wonder if you have access to the mathematical calculations that determine the relationship between visible
diffraction and sensor resolution. I have never seen this, and suspect that nobody can produce it simply because much of this is based on an urban myth:https://jonrista.com/2013/03/24/the-diffraction-myth/http://community.the-digital-picture.com/showthread.php?t=809
Diffraction in physics is caused by light passing through a slit or hole. This comes from Canon:
Light comes from the left, passes through the slit (indicated by the blue line), causing light to be diffracted (on the right).
The amount of diffraction cannot be altered by changing the size of the pixels on the sensor side. The width of the slit the light passes through is the only parameter which can influence the degree of diffraction. Reducing the pixel size on the sensor therefore cannot alter the amount of diffraction that occurs. For a slit of a given size, irrespective of the sensor resolution, the amount of diffraction as it passes through the slit is always the same. Higher resolutions/smaller pixel size may make the diffraction more visible, but it cannot alter the amount of light diffraction. So the idea that higher sensor resolution makes diffraction set in at ever wider apertures makes little sense.
Thus the idea that a future 120MP Canon full frame sensor on the FE mount might cause diffraction to become a limiting factor hardly makes good optical sense. That is why I think we need to see the actual mathematical demonstration that underpins these claims about the relationship between visible
diffraction (sufficient to cause IQ degradation) and pixel size. Until I actually see this, I am going to call the commonly repeated stuff about diffraction limiting the usefulness of high resolution sensors an urban myth until proven otherwise. I would be happy to be proven wrong.