This is the point where I can use some first hand knowledge. In 2003-04 I was living in CA still. I was talking to some parents whose youger son had played with my son all through Pop Warner and his freshman year of high school. There older son was at that time a senior at Wisky. When it was announced we were getting Cosgrove I said to him. Hey looks like we just stole your son's coach. He looked at me and said, "your team just got worse as a result." This was not sour grapes. Their son was a starter. His opinion which was backed up by his son, was that Cosgrove was not a great DC. Basically they said he could not spot trends and that often put them out of position.
Our team got worse the day Solich was fired, and even worse the day Callahan was hired.
I'm not claiming that Cosgrove was a defensive genius, and one anecdote doesn't change the reality that the defense was better than the offense. Here are the Big 12 conference rankings.
2004
Scoring offense: 7th (25ppg)
Total offense: 8th
Scoring defense: 10th (27.09ppg)
Total defense: 8th
2005
Scoring offense: 8th (24.67 ppg)
Total offense: 11th
Scoring defense: 4th (21 ppg)
Total defense: 4th
2006
Scoring offense: 4th (30.57 ppg)
Total offense: 3rd
Scoring defense: 2nd (18.29 ppg)
Total defense: 5th
2007
Scoring offense: 8th (33.42)
Total offense: 5th
Scoring defense: 12th (37.92)
Total defense: 12th
Pay close attention to 2005 & 2006 (as I pointed out in the post you responded to). 4th best scoring defense in the conference in 2005 (good for 25th nationally). 2nd best scoring defense in the conference in 2006 (good for 24th nationally).
The wheels clearly came off in 2007 defensively, but the offense wasn't lighting up the conference either (8th in scoring offense).
Cosgrove was
A problem, but he was not
THE problem.
As far as the offense goes, most people consider the 2009 offense to be one of the more pathetic offenses in recent memory. Believe it or not, that unit outscored both the 2004 and 2005 offenses.