I'm not sure how a younger Billy Sharp is materially different from Brewster, it's not like he's an Andy Carroll type who can hold the ball up. He's also a short poacher, the difference is he's capable of heading the ball, setting up goals and fitting into Sheffield United's system. As for creating nothing, they created more big chances than Wolves or Palace that season. Still bad, but if you have someone better than Brewster or McGoldrick to lead the line you start turning 0-0s into 1-0s.
And the reason he never got a run was because he played horribly and contributed nothing. Obviously he'd have scored more at Swansea, he would have been in a league where Dominic Solanke, who did only marginally better in the top flight, just scored almost 30 goals in a season.