If hagler had fought 8 -9 times a year like the fighters before him, how long before he wore out? Say he defended his title 2 or 3 times then the others were nontitle fights like was done even through the 70s. Would Marvin Hagler last with this kind of activity? Monzon fought many nontitle fights. So did Duran, Galindez, Napoles. Say the money forced him to. How long does he last?
Hard to say, it may have cost him in the long run with burn out. As you say, some fighters seemed OK and old timers thrived on it. Personally I think, if he faced at least 8 challenges a year, then by 1984 we'd be looking at him losing.
As you say, he did fight very regularly throughout the 70's but as he famously said... "It's tough to get out of bed to do roadwork at 5am when you've been sleeping in silk pyjamas"
One of the reasons he lost to Leonard at a semi young age is because his fight output dropped dramatically in his last few yrs and he got rusty IMO
Hagler's a guy who I think probably could have handled that physical workload as he was always in shape and seemed ready to go. Hopkins is another guy in recent years I've seen with similar levels of dedication. Do I think he'd have gone undefeated in non-title fights at 5 a year? Maybe, maybe not. Do I think he'd have continued to successfully defend his title 2 or 3 times a year like he did between 1980 and 1984? Yeah, I reckon he would have. Maybe the intensity he brought to his title defenses would have been adversely affected by fighting those non-title fights but I personally think Hagler had too much pride to lose that title during his peak years. Assuming he fought 5 or 6 non-title fights a year between 1980 and 1984, that's 25 to 30 fights on top of what he already had. Hagler with a career record of 92-3-2 with those extra fights being at a 'keep busy' challenge level? Yeah, I can see it.
The rust did hurt Hagler a lot. The amount of fighting he was doing and his motivation was not of an elite fighter in 1987.. And Ray knew it and also knew that beating Hagler regardless of Hagler being diminished is still a win over Hagler.