There's another issue which hasn't been mentioned so far, and that's the effect that increased pixel quality and quantity has on lens performance and effective focal length range.
Let's take the example of someone who thinks that the 8 megapixels of the Canon 20D is good enough. That's a camera that I was very impressed with a few years ago. The pixel quality and high-ISO performance was a major upgrade from my 6mp D60, and the 8mp were certainly good enough for A3+ size prints.
Let's consider the case of someone who is still using the 20D today. He can afford to buy a 50mp 5DSR but sees no point. He thinks he doesn't need the extra resolution, and he's not interested in video, but he is interested in various type of photography, and has a variety of Canon lenses ranging from wide-angle primes to telephoto primes and zooms.
Now it so happens that the pixel quality of the 5DSR, according to DXOMark, is approximately as good as the 20D pixel, despite being smaller. Dynamic range at all ISOs is identical for both cameras at the pixel level. SNR at 18%, Tonal Range and Color Sensitivity are just slightly worse for the 5DSR pixel, but not to a significant degree that would be of concern to someone who considers 8mp to be good enough.
The advantages of the 50mp 5DSR in relation to either a downsampled 8mp image or a crop to 8mp are just amazing. For example, a 50/f1.4 lens on the 20D is effectively a fixed 80/F1.4. You can't get wider without taking multiple shots for stitching, and you can't get narrower without sacrificing pixels. However, on the 5DSR, that same lens becomes a 50-125mm zoom with a constant maximum aperture of F1.4, in relation to that 'good enough' 8mp standard.
The 100-400mm zoom becomes a 160-640 zoom on the 20D, taking into consideration the crop factor. On the 5DSR the crop factor of 8mp taken from the centre of the frame is 2.5x. In relation to the 8mp standard, the 100-400 zoom becomes a 100-1000mm zoom. Wow! What a lens!
Which would you rather have? The extra weight is a mere 100gms or so, and if you're worried about being able to see the subject through a 400mm lens which is effectively 1000mm, there's the LiveView facility which the 20D doesn't have.