http://hitrowz.com/2013/07/14/the-myth-of-the-20-goal-a-season-forward/
Very interesting article that makes a lot of sense. As devil's advocate though, lost in the statistics would be instances of where players were injured and missed games. Also, the example of Crystal Palace's 2004-2005 season (of which I really didn't follow them), and being relegated despite having a 21 goal scorer, could also be the explained due to other factors such as injured players, poor coaching, and/or a bad mix of players.
In a sense, it's somewhat similar to an NBA basketball club with a 30 PPG scorer yet having a losing record. Having a 30 PPG scorer on a basketball team does not mean that the team will have a winning record --- and the fact remains that on even the worst teams, someone will be the 'designated' focal point to score. But if they were on another team, they may just be designated as a utility player/sub, who may not be given the same scoring opportunities that they were given on their prior losing club.
All in all, still a very interesting read with some great points.