Leonard was effectively retired after the Hagler fight, or so he said ('This is not a career, this is one fight.') Of course, Leonard's 'retirement' can be taken with a pinch of salt and he was back in the ring the next year against Donny Lalonde.
He didn't effectively retire after the Hagler fight. Leonard announced his retirement after he lost to Duran the first time. He announced his retirement after he beat Bruce Finch. He announced his retirement after he beat Kevin Howard. He announced his retirement after he beat Hagler. He announced his retirement after he beat Duran the second time. He announced his retirement after he lost to Norris. He announced his retirement after he lost to Camacho. Leonard announced his retirement all the time. In his last 13 pro fights, he announced his retirement after seven of the 13 bouts. Top Rank expected a Leonard-Hagler rematch. Leonard expected a Leonard-Hagler rematch. Hagler expected a Leonard-Hagler rematch. But after Marvin pocketed $18 million for the first Leonard fight, he filed for divorce and moved to Italy to make movies. He wanted to be an action star. Everyone expected the rematch would be signed when Marvin's divorce was final. After it was final, Hagler said he was going to stay retired. My point is, Leonard didn't avoid a Hagler rematch. Hagler pushed it back (until after his divorce) and then turned it down. That's why Leonard waited so long into 1988 before he fought Lalonde. Everyone was waiting on Hagler. But he decided he didn't want to fight anymore.
Monzon's title opposition tends to get overrated for me but having said that i rate him a miniscule above Hagler mostly based on the last fight of both their careers with one riding off into the sunset after beating the second best in the division and the other losing to a guy that hadn't fought in years. If Hagler beat Leonard even reasonably impressively i have no idea what i would do.
