John Wall has fell off in a similar fashion. Both their games were built on speed and their explosiveness. Westbrook was a lot of drive and kick to make the offense run. His speed and explosiveness was what created the gravity for him to orchestrate the offense.
Guys like Westbrook and Wall really have a limited window before the 82 game NBA season really starts taking a toll on their body. Westbrook carried a huge load after Durant left OKC. He probably might have had something left in him if he didn't average 36 minutes, 80 games, and a triple double for 3 seasons with the Thunder