Most fighters from past eras fought more often and lacked the advances that medicine and experience has taught us. If the likes of Marciano, Dempsey, Ali or Armstrong fought in this era they would be heavier, probably quite a bit heaver, would fight less and would be as good or indeed better than anyone in their weight classes. That also helps explain why fighters seem to go on for longer than in the past.
It's not a good question IMO although I understand why people ask it. Boxers are precision athletes at the sharp end. They're well trained for what they do whether that's an extremely low workrate over 40 rounds or a high work rate over 12 rounds, or whatever is in between. I do think that there are advances in nutrition etc., that have definitely helped modern fighters though. Not using brandy to "fortify" a fighter is one example of hideous science that has been banished in more enlightened times.
15 & 20 year careers T and fighting much more frequent IS a helluva long career. how is 12 - 15 year careers fighting infrequently longer?
NO:-(, The old timers were a lot tougher, stronger, more stamina, grit, had more tenacity, and more will to win, and they didn't have peds, creatine, roids, and all of that other hippie, dippie, bull****... Plus they fought on steak and eggs, chopping trees, and running on pavement , with combat boots on, and fighting 30 , 40 rounds, with half their face falling off before it was stopped:deal Guy's these days have no balls, they pad their records, fighting taxi drivers, and bums, forklift operator's , tomato cans, holding out for the big fraud fight, then get caught juicing, they fight 12 rounds at most, and sometimes the POS, ref if a guy gets a papercut...
Like all things, it's a matter of the "haves" and the "have nots". The big time millionaire boxers who have full-time strength and conditioning coaches, dedicated sports scientists, and personal chefs certainly will have a fitness advantage over boxers from way back when. But your average up and coming boxer is probably not training much differently than boxers have been training for the last two hundred years - lots of running, lots of hitting stuff, lots of calisthenics and varying amounts of sparring.
I don`t think modern fighters are in better condition than fighters from past eras. I actually think the reverse may be true. Fighter may lift weights more and in some cases be more heavily muscled. That is not a true indication of condition.
This is one where the old timers were definitely better. Simply can't compare to a high paced 15 rounder. Knowledge and its acquisition are better. But knowledge does not mean application.
Modern heavyweights are fat. Heavyweights before 1960's weren't heavyweights by modern standards. The heavyweights between 1970's to the early 2000's were in better shape compared to current heavyweights. In other weight divisions the boxers are in basically the same type of shape.
Not really if you look at a sport like base ball where the bat is still made of wood like it always was many of the longest home runs were a long time ago. https://syndication.bleacherreport.com/amp/2698852-the-longest-home-runs-in-mlb-history.amp.html
