As other posters have pointed, it is not like Marvin gave a lot of space to Tommy. It was "fight or run for your life" and Tommy didn't run. Just two cents: 1) For a good part of the brawl, actually it was Tommy the one coming on top. 2) Nobody knows per sure what would have happened if Tommy didn't break his hand.
Yes if he didn't break his hand. And managed to recuperate and work on Marvin's cut. Would have been awful for him to lose his title on a cut but Tommy would have gone for it.
I genuinely think he thought he could knock Hagler out and I'm glad he went for it because it turned out to be the best 3 round fight in the History of boxing.
Remember a boxing article where they interviewed the Hitman and they asked him why he traded with Hagler and he said he kept on running in and he did so because Hagler showed no respect .