In some reader polls taken between Montreal and New Orleans, he was rated the second greatest WW of all time after SRR, and P4P #2 ATG. He'd be the generally accepted P4P #2 if Montreal was his finale. He was my own #2 ATG behind SRR until I bumped Greb up to number one. (The clincher for me was the superiority of Duran's performance over the first 12 against peak MMH versus SRL's inferior showing with a slowed Hagler, and how Duran redeemed himself for Hearns by defeating Barkley in The Blade's career best display between Iran's wins over the Hit Man. Personal preference. It was speculated in KO before Montreal that Duran "May be invincible when in prime condition," which I believe may have been correct. He stops Hearns after Montreal in a WW unification before Tommy peaks at 154, and that's what he should have done legacy wise after Montreal. He took the eight million for New Orleans though.)
Of COURSE his all time ranking improves. He would retire with only one loss he avenged TWICE BY KO. His legacy would be a fighter that was practically unbeatable.
The guy is generally considered to be either #1, or 2 at Lightweight ever, and as high as 3 or as low as 7 p4p all time great. IMO he could never come above SRR in ATG rankings anyway so it makes little or no difference whether he had retired after Montreal or not. It is merely a case of whether or not you think he could have made #2 after Robinson.
I see a fair bit of potential for difference, based on your own comments. There's not much difference for those ranking him 3 already (tho 2 is certainly different to 3) but what about those ranking him 4-7? He could come down a number of places. When e are talking top 10 ever every couple of places is massive. He could have become a consensus 2-5 say. Of course boxers rarely retire at such stages, it's mostly tragedy (Sanchez) or injury or incarceration.
As I mentioned I think if he quite after the Leonard fight you`re looking at a top 2 ATG fighter, rather than the 5-7 most have him at. More importantly you take away the `no mas`argument that may use to degrade him, either in ranking or h2h hypothetical match ups. Great question btw.
Number 2 of the more modern era would certainly be defendable i reckon. He'd have one helluva reputation.