For me what separates Duran from Canelo, is Duran being the most skilled fighter I've ever seen. If anything, I actually think their level of athleticism is closer to their level in skill; although obviously Duran was far better in both areas.
I don't think Canelo is very "Skilled"- he's adept, has a plan, and works his ass off between fights. He plans out each fight, each battle, very carefully in advance. Duran is the natural.
Assuming both were in fighting shape and had to take on a tough opponent with a few day's notice Duran would nearly always do better. Like you point out i think Canelo works on things and gets incrementally better each time out. Duran is just a more natural fighter.
Duran was an incredibly natural fighter. Just watch him vs Kobayashi, before any great trainer started to work with him. I do think Canelo is highly skilled. He's a great ring general and has one of the best arrays of feints in boxing right now. He's a great counter-puncher, throws mean combos and has excellent form. Defensively, he's mastered neckrolls as well as basic head-movement drills. The only issues he has in terms of skillset is that he hasn't got a steady, varied jab, and hasn't got much in the way of infighting.
Duran is THE most natural fighter of all time, I think that's very safe to say. Irish made a good point last month when he said Duran's ability to fight off-the-cuff separated him from a more regimented guy like Chavez. Canelo is more Chavez than Duran.
True, all i would ad is once you get to the very top level it doesn't matter as much. Not only is nearly every fight available to see with a few clicks, fighters have months of preparation.
I know the running joke is the whole 'Duran is better at everything than everyone' schtick, but he really is the most complete fighter ever IMO. There's no category in any facet of boxing where he's anything lower than an 8. The skillset he had, and how he managed to transfer it into various different styles throughout his career prove this IMO.
The most complete fighter doesn't necessarily win though, sometimes a super-talent in one area beats better overall skill. Pernell Whitaker wasn't a 'complete fighter' the same way Duran was, but many, including me, would pick Whitaker h2h
I went through a period where I said Duran-Whitaker was close to 50-50, but I no longer think that. I think Duran would have beaten Pea clearly.
I think Wilder before the fury beat down was considered p4p by Akhi, Dontae and Ego. And I dont believe he boxed as an amateur. Im not sure he knows how to box now but has power.
Wilder was never pound for pound, and won a bronze medal at the Olympics. Safe to say he doesn't qualify IMO.