Riley is tough to judge because he was handed a well oiled machine that he had already been involved with. Easy transition and he's done well. Kiffin was a bad hire because he hadn't proven that he could actually be successful at a Power 5 school and I don't think he was ready for the limelight. Sarkisian might have been good....if not for the alcohol issues. Helton just didn't have the personality to keep good talent in LA and the talent drain that had been going on was too much for him to overcome.
Riley was 55-10 at OU with conference titles in his first three seasons and NY6 bowl appearances every year that he was coach....well, except maybe this year. Riley might have ended up being more like David Shaw at Stanford. Harbaugh had everything going well and Shaw kept that going for three seasons before things started to drop off. Stanford had a nice dead cat bounce in 2015 before steadily declining since then. Would that have been Riley's future at OU? We'll never know.