I can see your point,modern fighters do have it better in so many ways. In general over all the weights I think the fighters of old were more mentally tough from the work during the times and were more determined grittier fighters that could go for some amazing number of rounds.Most fights were for around an hour but varied from 20 minutes up to 3 and half hours. One bare fisted boxing match went for 3 hours 16 miniutes after which one fighter Simon Byrne died 3 days later (Byrne had also killed a man in the ring prior to his last fight.)
Theres the difference; skills correct diets advanced techniques lesser rounds and gloves.
Cant compare the two really and then there s the guys who fell into the game in between the two extremes some had some of the old grit in them some more of the newer stuff some a touch of both hard to draw a distinct line in the sand though. 20 rounds,15,12,10. You take one fighter from here to there or one from there to here and each may not do so well in the others eras.