It's not the leaving as much as it is the way he left, as well as the lies he continues to tell about his leaving.
We'll see how he does at USC. If he's the great success that Colin Cowherd and Pete Fiutak think he's going to be, and if Brent Venables turns out to be in over his head as HFC, then maybe we'll wake up and wish we had kept him. But right now, 99 and 44/100ths percent of Sooner fans feel like we got an upgrade.
As I was pointing out long before he left (albeit maybe not on this board), he was getting a little-less-great results with each passing year. I wasn't calling for him to be fired, but I feared that there was not going to be a turnaround any time soon.