My take on this fight is that it'd be a fairly boring, tactical fight. Sharkey never fought a southpaw to my knowledge, and if he did, they certainly weren't good enough to prepare him for Usyk. But Usyk had some issues with Hunter's reflexes, so I can see Sharkey's head-movement giving Usyk issues. Usyk figured Hunter out before Mike gassed though, so I think he'd do similar here. Usyk takes a decision. Having a much higher workrate and better track record doesn't hurt either. There should be absolutely no question about whether Usyk can go 15 or not. He threw 900+ punches against Gassiev, and moved every minute of the fight and wasn't even out of breath by the end. His conditioning is nothing short of phenomenal. Usyk UD12/15
It'd be also interesting to see what approach would Sharkey take against bigger Usyk. I think he'd use a lot of head movement and he'd use his crouch more often than, let's say, against Schmeling. Sharkey was dangerous pressure fighter, but he lacked power to hurt Usyk. I agree with George here, Usyk by decision is a very possible outcome.
Usyk is a very consistent boxer with near pound for pound skills who can take a punch. He also has more height 6'3" and reach here...by a good margin. Sharkey was not consistent and had significant issues taking punches as he was down at least 15 times in his career. Usyk UD or later round TKO. Many wonder if Usuk can beat a super heavyweight with skills. I'd give him a shot vs. both Fury and Joshua, who probably will not fight Usyk until Usyk ages a bit more.
The résumé question is interesting. Sharkey has more depth, but Briedis and Gassiev would slaughter everyone who Sharkey legitimately beat.
Everyone Usyk for the last five years is bigger than Sharkey. And I'm 100% positive that Sharkey isn't gonna hit him harder than Gassiev or Briedis did.