I don’t think Hearns really could, Hagler was all over him. When he did try to box more and use more lateral movement in the middle of the round, Hagler cut him off and got him in the ropes. When Hearns used more backfoot movement after the 1sr round, Hagler kept getting to him and tagging him cleanly. Hagler had to close the distance due to Hearns being so great from the outside. Not only was Hearns extremely hard to box at long range, but his right hand was most dangerous when he got distance and more leverage.
That fight was a war for many reasons. Hagler fought unusually aggressively, inconsistent with his usual style, the result of it being his biggest payday to date, with the most hype of any fight to date, plus all the trash talk, plus his belief that a blown-up welter who got stopped by Leonard could not handle his power, and that his chin and defense could handle whatever Hearns had to offer. BUT, it also was Hearns who willingly decided from the jump to throw bombs away nonstop in a relentless pace, going for broke, rather than pick his shots, use some lighter set-up punches, his legs, and clinching (which he rarely used and never was particularly adept at doing). He basically quickly punched himself out. Hagler correctly believed that relentless nonstop pressure and getting to the inside quickly would take Hearns out of his comfort zone, because Hearns wasn't really an inside fighter. Hearns figured Hagler would walk right into his bombs and he was happy to oblige, thinking that his power would be too much for anyone who came to him. He was wrong. Hagler was right. I'm certain that in a rematch, Hearns would alter his style, box cautiously, pick his moments, pace himself more, and clinch more on the inside. His legs wouldn't be gone so quickly because he wouldn't throw so many hard punches in one round. He would make Hagler come to him, use his height, reach, speed, and timing, and sniper him. Would it change the result? Doubtful, but it lasts a lot longer and there might be the aura of doubt about a decision as the middle to late rounds arrived. But I think Hagler eventually still would have gotten him.
I thought that too, after about the 1st 90 seconds when Hearns had thrown his load and was now desperately trying to keep Marvin off him that the fight was going to end one way only. Hagler by brutal KO.