boxing is primarily a cardio sport, and your cardio is best in your teens and twenties. cardio falls off with age especially from 30 onwards, perhaps a little slower for some, but it falls. you are going to be prime in your late twenties to early 30s, when experience gain and cardio loss eclipse each other the most. Hopkins was prime in his early to max mid30s. without his cardio of his 20s/30s, he lost to guys who he was clearly outboxing like joe calzaghe.
Well the thing about boxing is that I cannot find a sport with the demands that it has. Power, speed, stamina, toughness, skill are all very important with the possible exception of some guys can get away being slow like Monzon and Arguello. Other than the rare exceptions you needs basically everything. But I would agree that stamina, gun to my head, is the most important thing in boxing. Having boxed myself I know what it's like to gas out. You're screwed when that happens. So your point is good. Hopkins workrate declined because his stamina went down.
Hopkins seemed to have the more workrate against Pascal, Pavlik etc. Most of his fights he seems to have finished the stronger.
yes true, Hopkins workrate steadily declined as he aged, cant believe some fools pretend its different for him than everyone else in the universe.