Agreed, too many people get caught up in the Tom Brady story - "A sixth round draft pick won 7 SBs, so we can take a 4th round lb, de, qb, etc. and make him into an every year Pro Bowler". Or "Give player X a couple more years and he'll develop into one of the top players in the league at his position".
In the first instance, talent can't be hidden very often. If a player has the requisite speed, size, quickness and most of all quality college game tape he WILL be drafted in the early rounds the vast majority of the time. "Hidden gems" aren't really as common as most think.
Second, seldom do average players "blossom" into top players after 3-4-5 years of being backups or part time starters. If they can't learn the game in the 3-4-5 years MOST of the time they can't ever get to the top level.
Good drafting, good coaching, good cap management and (yes it's real) some luck wins SBs, not making a "silk purse out of a cow's ear"...