Here's my take on it: Birmingham went down because they couldn't score goals. They prospered last season by winning a lot of 1-0 matches. That was never going to be sustainable, but the blueprint was there. Be a difficult side to breakdown and do enough on the other end to collect the points.
Without McFadden there wasn't that level of guile, of experience, aggression... whatever combination of skills and traits that Faddie brought to the team. No offense to Beausajour, Fahey or Bentley, but they're not Premier League caliber starters yet, never will be, or never will be again.
So Birmingham were relegated with McFadden's absence for the large majority of the season only proving just how valuable he was to the team.