Westbrook averaged 8 assists per game in 2010 and 2011, last year happened to be a down year for him with assists but him averaging 8 assists doesn't tell me much.
It goes beyond Westbrook though and it's more about what's winning in the NBA. Teams that aren't relying on great offensive production from their point guard always do better in the playoffs, and the team that's going to be in OKC's way has proven the NBA is controlled by elite wing players right now. I'll take a dominant tandem at SF and SG over a dominant tandem at SF or SG and PG every time.
The way the shooting guard position has gotten smaller across the NBA is another factor. Most of the other big shooting guards (Kobe, Joe Johnson, etc.) are old and past their prime. When they're gone and Harden's compe ion is Bradley Beal, Monte Ellis, Eric Gordon, etc., he's going to completely own the position and be a mismatch on any given night. It would have only been a matter of time before Harden was gonna be able to get whatever he wants iso'd against Wade. Westbrook isn't ever gonna have that kind of advantage against Miami.
The Thunder were gonna win 57+ games regardless of who they kept between Westbrook and Harden, the difference is which one makes them a better playoff team. A shooting guard with a high bball IQ who's clutch as outside of a bad finals performance is much easier to have as a key ingredient on a championship team than a volatile scoring point guard.