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The high-MP cameras typically do offer some "sRAW" binned RAW function.
Nikon doesn't do any binning in its 36MP D800
However, because of the Bayer-filtered sensor structure, there's not much gain in file size until you drastically reduce the resolution. Full RAW data = 1 ~12-bit sample per pixel. Bin together groups of 4 pixels (for a quarter the pixel count) but keep full 3-component color at each pixel site, and you now need 3x12-bits for each of 1/4 of the original pixels (3/4 of the full RAW data size). In other words, file size goes down by only 25% to keep a "good" 9Mpx RAW image from a 36Mpx sensor. You can further reduce the file size if you bin...Show more →
I agree that binning is hard in bayer structure. What you have described is really pretty much like doing the demosaicing and then keeping RGB data so not really binning in RAW. I am not really sure if there is a right way to do it But the point is that its a huge waste to have to store and process 36MP RAWs if all you need is say 8-10MPs. If the future is all ultra-megapixels then there should be a way where you could choose to have a smaller resolution while still keeping the benefits of a RAW file.
Also I am not completely convinced on the "more pixels always lead to improved performance" paradigm. Yes it is true that sensors have been improving overall even with the increased MPs. However it is possible that if instead of spending all the technology on making higher-MP sensors work well if the same technology is used for improving the parameters of a lower MP sensors, the final performance in terms of things like DR, noise etc might be even better. Nikon did that with the 12MP sensor in D3s when they opted IQ over higher MPs. D3s sensor despite being much older is still better than D800 at high ISOs, the same is true for the 16MP D4 sensor. If Sony's advancements in sensor tech were applied to a D3s/D4 like sensor, the results could have been even better than what we get with D800.
Maybe at some point in future we will get bodies with swappable sensor modules and then manufacturers might be more willing to build different sensors with different priorities.