Most importantly, in 1985 Hagler still was motivated. In the 87 matchup, the difference in enthusiasm for the fight between Leonard and Hagler was pretty significant. Hagler had to be coaxed into the fight by his handlers, while for Leonard, as John Thomas has stated, fighting Hagler had always been his everest, and was something he had been planning for some time as he watched Hagler take punishment and fall out of love with the sport. Although a 1985 Hagler had already started to slide, his motivation was still intact, as evidenced in his fight with Hearns, with whom he fought like his life depended on it (the 1987 Hagler, against Leonard couldn't have been more different). A well trained, well enthused Hagler, who still retained speed, sharpness and stamina in 1985, would have ground out a points win. Probably 8 rounds to 4. I'm not sure even a peak Hagler stops Leonard. Perhaps over 15 he could have.
Leonard was no dummy. He waited until he felt sure that Hagler could be taken before he fought him. The Hagler that demolished Hearns would've been too much for Leonard and Ray knew it which is why Ray opted to wait awhile before meeting Marvin. Too many wars and advancing age made Hagler ripe for the taking and Ray capitalized on this. He was a shrewd businessman after all.
I also don't agree that Leonard lasts 15 nor close to it. Hagler's not going to be trading love taps with him. History has shown that if you deliver your blows soft and slow, Leonard stands a good chance of beating you. But the moment the opponent puts some speed and snap at the end of his punches, Leonard flounders. He always does. Leonard's going to get floored because that's what he does. His chin wasnt the strongest as we've seen by the many times he was floored. In addition, when he does take blows he tends to catch them flush (Leonard-Geraldo, Leonard Duran 1) that spells trouble right there. So how he survives the 15 is beyond me. I've also heard Leonard was much faster in 81 than he showed in 1987. But let's slow down and look at this a bit more carefully. On careful analysis, was he? that's not what the films show. The films show Leonard methodically laboring to a 9 round tko over Kalule, an ordinary 154 pound champion. Later that same year, the tape shows Leonard hopelessly behind on points and hopelessly outpointed by Hearns. How this translates to the kind of speed that would throw off Hagler also escapes me. That was the kind of performance that one is going to want to put in the scrap book.