PS. Ali was stopped only once, by Larry Holmes, and even then he wasn't down: His corner stopped the fight between rounds.
Ali has lost more fights than that. Frazier, Norton, Spinks, Holmes and Berbick have all defeated him, mind you the last 3 were more like beating up and old man.
Not to be persistent, but there were many fighter's in the 70's that would get very tired in the ring.
George Foreman, Ron Lyle, Ernie shavers, and even Ali during his later career.
However, while today sucks, during the 80's I haven't seen any footage of the top fighters ever tiring to the point of collapse.