Even if Conor bests Usman it wouldn't make him the greatest because outside of FW he's never had to actually climb the ladder in a division. He could knock out anyone WW and down on a given night but would you actually back him to put on a Tony like run through either LW or WW? Unlikely given how much he struggled with Nate.
Now when you are moving divisions as a non-champ I'm not saying you should start from the bottom but you usually have to have a couple of top contender fights first like Aldo vs Morais, Rockhold vs Jan, RDA when he went to welterweight etc. If he beat Masvidal and Covington first then it would be more legit.
You don't need to go through a division if you can take the head of the top dog immediately. It's an embarrassment and disgrace to a division when someone from a lower weight comes up and doesn't need to work his way up to be competitive with its very best... that, or testament to the greatness of the fighter who has gone up a weight class, took on its very best and prospered.
It's something every heavier champion should fear when facing a come up from a lighter division because, in terms of optics, it always looks absolutely terrible for the heavier guy, who just by size, should wipe the floor with this upstart.
Jones to Stipe
Adesanya to Jones
Usman to Adesanya
Khabib to Usman
Volkanovski to Khabib
Cerudo to Volkanovski
Conor to Usman; every single one of them would be disgraced losing to the one coming up, so for Conor it's an unfair, win-win fast track that would shock the UFC and send his hype machine into overdrive and certainly have him in rarely triumphant territory, traversing so many divisions fighting champions, especially if it then becomes 2 from 3 in victories.
For Usman, it's supposed to be a showcase of him putting someone he told outright to stay away from the 'big boys', back in his box and it will be the biggest humiliation he'll have in his career if he loses such a fight.
What doesn't seem to be accounted for is Usman has a massive ego and a lot of pride. He fought Covington the way he did out of principle not only to cause as much physical damage as possible, but to make clear he is the superior man outright. If he and Conor fight, I have no doubt in my mind he'll attempt to do the exact same and put Conor through as many tortuous rounds as possible... thing is, Usman's stand up is poor and he walks forward reliant on absorbing blows and firing back over avoidance and technical guile with craft. If he gives Conor the same openings and shots he gave Covington, he's going to sleep because they'll all be landing in the right spots at the right times with Usman's own size and momentum contributing to the impact point damage.
Not quite Aldo's levels of emotional irrationality, but along those same tracks of Usman wanting to destroy this guy will play right into Conor's only realistic inlet for the win. If Usman is cautious and looks to take him down, Conor will be in for a torrid night and gassed even quicker than normal, but the odds of Conor getting in Usman's head and having him coming out trying to decapitate him are a lot higher than a bigger man exercising caution in a fight he has said would be a massacre. The optics of Usman shelling up and potshotting shots would be terrible for him against the much smaller man who, he's already said, shouldn't even be in
his division.
Like it or not, 2 from 3 going up 3 weight classes would look tremendous on a C.V. even without the groundwork in the division. It'd also justify Conor being given preferential treatment, much to the ire of the rest of the star roster.