Hagler was not all that broad shouldered. He was a smallish middle with a small waist that made it appear he was broad shouldered. Look at Rocky Castellani or Eduardo Lausse for big shouldered middles.
Plenty could do it if they were allowed to land free shots on him. Nobody's chin is indestructible. The problem is, Hagler is a good enough boxer that he doesn't get hit often with flush clean power shots from bombers.
I'm impressed the more I see that shot....it was delivered with a whip sort of motion with the snap occuring exactly when it should have. It's amazing that Duran got up so soon afterwards, even if he was helped up by his seconds. Yeah, that shot could have decked Hagler.
Hagler would be on a pound for pound chin/durability list if there was one. There was actually some science behind his durability, and Hagler himself was very shy to speak about it. Hagler had a CT scan/xray after one of his fights. The Doctor said that Hagler had an abnormally thick skull that was 1 inch thicker than normal Jim Lampley once mentioned that Antonio Barrera, a fighter considered to have an excellent chin was determined to have an unusually thick skull after having his head x-rayed too. If you add in a strong neck and a tough mindset, you've got a fighter who's tough to hurt. Hagler once had a chance to move up to the light heavyweight division. He declined, so the upper limits of his durability are unknown. I think some of the best punching middles could take him out, but it would not based on just one or two big shots.
Would have to be a fearsome puncher... Julian Jackson, Nigel Benn, McClellan or Golovkin might be the three hardest hitters. Failing that a sharp shooter like Jones, Toney, Nunn, Robinson. I wouldn't favour anyone but if he was to fight all those back to back would anyone bet money he wouldn't be stopped once?