First of all, it's not as if Hagler had not crushed a tall challenger in title competition before. See Obel I & II. He atypically tore after Hearns and went immediately for the body, a much overlooked factor in why Tommy gassed so quickly. Take away the broken hand of Hearns, and the urgency inducing cut on Marv, then it lasts longer. However, with both men quickly exhausted, the far more durable Hagler secures a huge advantage. Marv did switch to orthodox during this one, but it proved not to be a key moment. He was clearly more effective as a southpaw, and Tommy was no Marcos Geraldo in adapting to the southpaw stance. [And Hagler-Geraldo was really the match Tommy and Steward should have been studying most for clues on how to try boxing Marv.] Hearns couldn't clinch, couldn't handle body shots with the resiliency of an SRL, and didn't have the chin to repeatedly absorb power jabs from Marv's right over a sustained distance. As skilled a boxer as he was, he wouldn't achieve full evolution in that area until Virgil Hill. Duran was surprised by Tommy's reversion to "Hit Man" predation, then Hearns in turn was stunned by Hagler's aggressive abandon, where Thomas was openly expecting Marv would actually be intimidated by what Hearns had done to Duran after Hagler's UD over the Panamanian. This instant pressing of Tommy by Marv would have always startled the challenger and induced a fatal attempt to try standing up to that surprise assault. Tommy needed to revert to Benitez-Sutherland-Minchillo mode to take this any kind of length, but he wasn't counting on Hagler tearing after him like that, and couldn't clinch as needed to neutralize and stall the momentum of that onslaught. Looking for ways that Hearns could survive to the final bell [let alone win] against an ATG opponent with that kind of motivation and surprise attack is a serious dilemma. Tommy simply didn't have the template to pull it off w/out Hagler utterly blowing it passively [like he did w/SRL, and nearly did with Duran, who both psyched him out, rather than up]. Place the Hagler of Scypion against that version of Hearns, and Tommy may have done well to get out of round one.
Something else to consider... I actually feel that Hagler was a little long in the tooth for the Hearns fight... It wasn`t a vintage Hagler,like against Sibson, Lee and Scypion. Age and tough fights will do that to a fighter... So, if you looked at true prime versus prime, I still think Hagler does it to Hearns, each and every time...That GRANITE chin of his doesn`t let hearns do what he needs to do... But it would be a war every time out