There is an incredibly easy answer to this.
The tipping point was the 2004 season, because by this point you had lost a lot of the Butch recruited AND COACHED players and you also lost Chudzinski as the OC. Coker wasn't perfect, but at least he had 2 UM alums as his OC and DC and once Coker started to lose coaches from the Butch era, he didn't replace them with equal/superior guys.
Also, by this point, you had a couple of significant changes on the recruiting side. First, this was the rise of the "internet ratings systems" and the Rivals/Scout class rankings. A lot of our coaches started to recruit from the office, instead of getting out and seeing kids in person. Also (as has happened at other times in UM's history), we had a lot of recruits who came to Miami EXPECTING to go undefeated, and who didn't understand how to work hard to create their own legacies. The kids who just assumed that they would be playing for conference and national championships.
By the time that Coker realized that things were getting away from him (including having to replace Greg Mark mid-season, though there were also some personal issues involved there), it was really too late. We had entitled players, we did not have the same strong coaching staff, and we didn't have anyone who knew how to restore things the way that Butch knew how to restore things.
Also, in complete honesty...2004 was the year we signed Willie Williams. Now, I don't hate the kid, but the way that he turned recruiting into such a clown show, with no real repercussions (we still admitted him) was a serious turning point. People want to yap about "Manny shouldn't have taken JT-4 back", but it was much worse what happened with WW.
2004. Willie Williams. Dan Werner. Entitled players. Seventh Floor Crew didn't happen until 2005, but the players and attitude that led to that was already in place in 2004.
PS - 2004 recruiting class was ranked #3 by Rivals, and then 2005 immediately went down to #10. 2006 class was ranked #14. 2007 class was ranked #22. Yes, Randy got us back to #2 in 2008 with a MASSIVE signing class, and we have not been back to the Top 5 since.