Sharkey took some punches, but removed his mouthpiece at "4" and managed to stay down for the full count. Perhaps I am drawing an errant conclusion, but it appears to me off the film that Sharkey decided he had gone far enough and taken enough punches. Honestly, I don't see any reason for him to have fought on. He didn't have the weapons to turn it around. Sharkey was never the puncher that Baer was. Or Schmeling. Many thought Baer could have gotten up, and Baer's own quoted comments seem to back them up. In fairness to Baer, though, he took at least as much punishment from Louis as Sharkey, and I think more. As for a younger Sharkey, I don't see him ever having the skills or power to handle Louis. I think he would need more snap in his right. Baer at least might have been able to pull an upset on an "on" night riding a big punch.
Valid points ... we are talking about Joe Louis and while he may or may not have been the best ever he's in that rare air so picking against him w a fringe fighter that had terrific and subpar moments is a bit of a reach of faith .. I happen to like Sharkey and feel the 27 - 29 version was a terrific fighter when he was on but who knows ?