Prime Joe Louis vs Prime Rocky Marciano

I gotta pick Joe as you can clearly see how visibly diminished he is from pre war era to post war era. However I will give Rocky some benefit of the doubt, since Louis did struggle with crouch oriented fighters. Schmeling and Godoy both gave good versions of Joe hell in their first attempt against him. However Schmeling countered more and Godoy was chin on your chest aggressive. Rocky kind of sat in the pocket a lot and Joe’s fast hands would bust Rocky up probably for a KO around the 7th.

 

Maybe a big reason older Foreman did so good despite having a large number of fights is because he didn't leave a lot in the ring with a lot of his fights, obviously Ali "aged" him some, his war with Lyle

There are variables, strongly correlated, confounding, and extraneous, unfortunately these are mostly one off examples of the absolute top tier fighters, in order to even start exploring the idea of causation there needs to be a very, very large sample size