I hate to compliment him, but Riley was an offensive genius and the best QB recruiter in the nation. It sucked going up against him, and we were at a disadvantage every year we faced him because of those two things.
It's possible his talent prevented a decline that should have already started. But it's difficult to compare OU's collapse to other programs:
Nebraska - incremental decline. They went from firing coaches that won national championships to firing coaches that won 9 games every year to firing coaches that sucked.
Florida State - Old coach lost it. Then new coach who wins a title initiates the decline.
Texas - Old coach lost it. New hires can't replace it.
Miami - Coach who wins a championship also initiates the decline. Lost its national image to the point where the brand they're known for only exists in documentaries.
USC - Begin to maybe fall off a little and coach bolts for NFL.
USC is the closest one I can think of. And OU doesn't have the same advantages USC has to make their down period somewhat moderate.