Hahahaha. I've always thought the more intriguing matchup would be Prime Foreman (when he demolished Joe Frazier) vs. Prime Tyson. Would it be like Foreman-Lyell? Foreman-Frazier? Or Tyson-Berbick/Spinks/Holme/any other fighter that ran into pre-Douglas Kid Dynamite?
As I've always said, this is a very close fight between two great, versatile fighters. Both guys are complete fighters. Hagler was a terrific boxer-puncher with a great jab, good combos, heavy hands, good set of legs, terrific chin, nice parrying skills and head movement, and he could slug it out as well and fight on the inside. Hopkins is a master of controlling the pace and picking his spots. He knows the angles and knows where to position himself to land good shots and avoid the incoming. Like Hagler, he is versatile. He can box from the outside using the ring, or maul guys up close and "stink it out". He has a big of dirty tricks to frustrate opponents. As proven throughout his career, he is very effective against lefties. Hagler had the heavier hands, superior jab, and was more proven in the slugging/pressure department. Hopkins was a little quicker and slicker, and just has a brilliant fighting brain in there. No matter how the fight takes place, I see it being very close. I doubt either guy gets stopped. Hopkins is slick and has a sound chin. Hagler's chin was great, and he didn't exactly have a soft body either. Over the years, Hagler was developed a reputation as a face-first brawler from people who don't watch enough of his fights, but he usually boxed behind his jab and broke guys down over the course of the fight. It'll probably be a pretty tactical fight, and I see Hopkins winning a close decision, with his straight right hand being the key.
I've said on a couple of these threads that I see Hopkins' advantages in speed, footwork, and countering playing a slightly bigger role in this fight than Hagler's better power and superior jab. This fight is just impossible to really say, both were so complete and versatile at their best, but I lean towards Hopkins to be in control more than Hagler in a very evenly-matched fight.
Hagler went life and death with every great welterweight he ran into and lost to one of them. Hopkins destroys any WW you put in front of him, PERIOD. Hopkins wins, and he makes it look easy. Hagler is wildly overrated.
I think there are many people who have set views of both men that don't really reflect what they were like in their primes. As you said, Hagler was a brilliant technical fighter, as with Hopkins, he could do everything in his prime and tactically and systematically broke his opponents down. Tremendous all-around skills and a great tactician. Hopkins is now viewed by people who haven't seen most of his fights from 10 years or so ago as a patient mover with a low workrate. In his prime Hopkins could be brilliant as an aggressive and busy fighter, wrecking his foes, or be a calculating genius like he is today with the kind of workate that he showed against Pavlik. Both men at their best were as complete as you would want a fighter to be.