Great post. Fact is, it is harder to compare eras than one might think, because folks today often forget how the playing field has changed. You put Jesse Owens on a modern track instead of dirt, with the shoes they have today, with starting blocks, and I'd bet he'd be right up there. Plus also keep in mind the timer technology today is superior.
He jumped 26-8 on a dirt runway when he was 21 years old. Imagine if he were allowed to mature to 28 or so (the age Mike Powell set the current record) had a modern runway (a huge, huge factor), modern sprint training, plyometrics, strength training, modern shoes, modern runway board, specialized in a single event... it's mind boggling how far he could have gone. (apologies for the derailment into athletics. it's the only sport i was ever decent at and i think applicable to some of the arguments made here)
I may be mistaken but non equipment sports, or rather basic equipment sports, i.e, straight up weight lifting and shot putt, have only minimally overtaken the records set by their supposedly archaic counterparts as there hasn't been much development in the equipment. Bit vague I know, just something I remember reading, but not the intricacies of the argument, hopefully someone can elaborate and make this something more than a waste of thread space.
My theory .... Probably the drugs. The drugs that track and field athletes were taking by the bucket load in the 1960s - '80s (basic anabolic steroids, testosterone, dianobol etc.) are hardly used now because of strict testing. The new undetectable drugs are great for sprinters, and all athletes, but the big throwers and weight lifters who develop maximal strength to obscene dimensions still do well (better) on the old drugs. The testing has made it's impact most on these disciplines. Everyone still does drugs but not lots of substances are off limits. It's just a theory mind.
I think John L. Sullivan would beat most of the fighters in the 1920s. I suppose, prime-for-prime, Dempsey would be the only one who'd be even-money against him. Tunney and Wills and Sharkey and Godfrey would be threats but slight underdogs I reckon.
The way I see it-and I could be tortuously, ridiculously wrong so if someone more knowledgeable disagrees assume they are right and I am wrong- the old heavyweights are almost directly analogous to the next generations. Sullivan is Louis with the tremendous record and the questionable opposition; Jeffries is Marciano (although the size factor is switched) considered great solely based on their unbeatablity in their short primes; Johnson is Ali a brash and hated pioneer (and jackass) that garnered near universal recognition as the best; Dempsey is Tyson all myth and destruction, and damned good fighters to boot, guys who demolished their opposition and left no doubts. Anyway that's just how I think of them and of Sullivan was as good as I think he was and does just half of what Louis did transported tho ths '20's he's an all time great.