So I am as guilty as anyone going through these new bodies trying to see what exactly recent improvements mean for photography in general and my photography specifically. But 10 yr old cameras could take good pictures, anything within the last 5 years likely had sensors with better noise control. Which camera you need really gets very specific.

1DX III vs R3. I agree, I would likely go R3 at this point. But what if I was a shooter than needed the same memory card in each slot? Then 1DXIII is the choice. While specific, I have seen a number of videographers state that they will not be buying the R5 for this exact reason, as they are shooting they want to seamlessly transition from one card to the next. Those videographers/photographers tend to be using Sony or the 1DXIII. 16 fps mechanical vs 12 fps mechanical...both very fast, but I've seen a pro sports photographer assessing rolling shutter on the R3 'cause any distortion is not going to work for him/her...answer, 1DXIII's 16 FPS is still better.

Then, there are still some details I haven't been able to track down. For example, in promoting the R3, Canon mentioned how the AF is calculated 60 times/sec compared to the R5's 20 times per second. I haven't seen frequency for AF calculations through the OVF of the 1DXIII which as a dedicated sensor and processor for AF. Other details, but mirrorless sensors are always on, will this cause them to heat up? I have always noticed more dead/hot pixels after extended liveview usage. What happens to mirrorless after 4 hrs at a sporting event? Maybe that is under control with mirrorless and it is a non-issue as I have not heard anyone comment on this. However, going through R5 files, I do occasionally find a noisy image that I wasn't expecting (ISO 800-2000).

But, IMO, it is very specific details such as that. These are elite, highly capable, extreme spec'd cameras. Which elite camera will work best for you and your very specific situation? Which specific features would benefit your photography?