Of course Calvin Johnson by himself hasn't made the Lions a winner (neither will A.J. Green alone make the Bengals a winner), but the Bucs drafting Calvin Johnson clearly would have made them a better team than drafting Gaines Adams.
I agree with your last point, though - better to have the infrastructure in place, at which point positions like WR can be filled fairly easily. Maybe not with superstars, but with players like a Garcon, Collie, Branch (in his prime), Welker, etc. As I mentioned, I wasn't thrilled about any of the players the Bengals had to choose from at four, but I'm really glad they didn't reach for a QB and think Green was the best choice of the available options.