Imagine if Thomas Hearns had an iron chin to go with his height, range, boxing ability (He was outboxing Leonard in their first fight) and power. Who, if anyone, could have beaten him?
It was foot movement, usage of the ring and angles that troubled McCallum greatly (he would've lost to the Nunn who fought Tate or the Eubank who fought Benn) - Tommy was more planted, trying to keep an opponent at a distance while stopping in the same spot a lot. Mike counters his long leads below the shoulders and follows with right hands to outwork him.
He likely would have finished the first Ray Leonard fight and probably would have won... He might have out survived Marvin Hagler, given how badly Marvelous was cut early on... I also doubt he drops a loss to Iran Barkley... In short, he probably finishes as a p4p giant.
in the long run Tommy is greater than Roy and more iconic. Tommy came to fight and was in great fights.
I don't think you can have a guy built like Tommy to have a great chin. The height stretches him out and makes it so his body cannot absorb the punch like someone like Hagler, who is more compact. Any tall fighter is going to have problems taking punches.
Still very serious questions about his resistance to body work. SRL bucked him badly with body shots at a key moment during their WW unification match, and he quickly gassed in part against Hagler because Marv didn't even acknowledge him to have a head to target in round one. Tommy was really a front runner who lacked the experience and then resilience to weather thrrough adversity. Always a first ballot HOF lock, he was nonetheless also recognized by observant scribes as having a ceiling on how high he could climb [even in the immediate aftermath of Duran, with the specter of SRL I haunting him]. At WW, he may have been fortunate to achieve his rise a few years after Palomino lost that title. Carlos had a granite chin, a deadly hook to the body, definitive 15th round knockout power [a round in which he decked and fractured the jaw of uber tough Armando Muniz], and he could one punch his way to a count out win after being dominated [Boy Green]. Palomino-Hearns would have been fun, and having an iron jaw may not have benefited Tommy one iota here. [Take a look at Palomino-Stracey for some evidence of this.]