Ali lost a decision to another unbeaten champion and won 6 rounds. It was as forgiveable a clear loss as they come.
I'm a big Frazier fan, but I wouldn't say he "beat the breaks" off of Ali in '71. Joe was in a lot worse shape than Ali after the fight
He got beat by a fellow top 10 ATG heavyweight whom was also in his prime. And Ali made one hell of a fight out of it, check out the firepower he unloaded in the 1st 4 rounds. No heavy today could take that, much less win after the fact.
The existence of zero losses on any fighters record after the completion of their career triggers immediate suspicion and demands heavy scrutiny and analysis of their records. Unless fighting during an exceptionally weak era, a fighter who duly engages the best around him, as and when he should fight them, will incur a loss here and there - no matter how good or great that fighter is/was.
I honestly think that people overthink "greatest" in Ali's case. Being the greatest is a state of mind, which he channeled to his advantage. He was a great fighter, who was up against other great fighters, and any lack of self belief could have been fatal. In the end it turns out that he perhaps was the greatest. In retirement, he didn't engage in the same sort of self promotion as Floyd Mayweather, he respected his opponents.
Ali proved he was the greatest Heavyweight in history by beating the best competition in history. Facts. Liston,× 2 Frazier, x2 -1 Foreman, Patterson, Norton ×2-1 just that group of fighters alone are in most knowledgeable fight fans top 20 in heavyweight history. 3 of those opponents he beat is routinely in most fans top 10 heavyweights in history. Factor in some of the greatest punchers in heavyweight history, fought in many countries, and at the least 3 of the top 10 greatest heavyweight fights in history. Not to mention being the underdog in two of the top ten greatest upsets in heavyweight history. Those facts alone speaks for themselves. Not to mention he fought EVERYBODY who was somebody during the greatest era in heavyweight history. Ali didn't win them all ? Of course he didn't because he was facing live bodies almost the entirety of his career. No Connor McGregor ' s , or " Name" fighters that were at times 10 yrs past their primes , you know like Deloyhoya, and Mosley was . Ali said on many occasions he actually didn't believe he was the greatest boxer in history, he always took a knee to the original " Sugar Ray" But he is much closer to that original "Sugar Ray " than that fella today who says he's the greatest simply because his record is 50-0. That fella is the greatest manager in history. Because he certainly made the most money for the least risk in boxing history. That he can claim to be " The Greatest " without much argument.
Frazier beat him when it mattered the most (both undefeated and closest to their primes), plus getting the only knockdown of their trilogy. Ali edges out the fighters who beat him due to his resume giving him a legit claim to greatest HW. Other greats you could argue as the greatest such as Louis or Lewis is because of the same reason and not the showboating. Ali as P4P greatest is something occasionally mentioned but you see the likes of Robinson, Armstrong, Duran placing higher usually.
When Ali said “I am the greatest” it was a war cry and the he actually went after the big fights and he did give great performances. With a lot of fighters claiming “the greatest”, P4P #1, TBE, it’s just a smug statement.