45 wins. Westbrook-led OKC won't win anything.
They went 3-7 this year in games Durant missed.
Last year, they went 26-29 in games Durant missed. They missed the playoffs because of it. I'm bolding that, because it's a big one. The Thunder went 19-8 last season in games Durant played (their typical 57 win pace). But because he missed a large chunk of games, they failed to make the playoffs. Think about that. They literally couldn't make the playoffs without a full season from Durant. We basically have our answer.
Over the past two seasons, the Thunder have gone 29-36 in games Durant has sat. That's a 36 win pace.
I think you're off a little here.
In 2015, the Thunder went 27-28 w/o Durant, not 26-29. They were also missing both Durant and Westbrook for 15 of those games, and Anthony Morrow was the only guy who played more than 70 games for the Thunder that year. That's pretty huge. Similar in 2016, while they were 3-7 w/o Durant, 2 of those games they were missing Westbrook as well (both late season when they were resting players).
In 2015, the Thunder went 22-18 (55%) with Westbrook and without Durant. I personally like the Thunder without Durant better than Portland without Aldridge (54% this year) and this at least is shown in the Win % area for these limited samples (though of course Portland did overachieve in most people's view this year).
I also think there's a difference if Durant leaves in the beginning of July vs if he theoretically went down for the season on November 1. The Thunder would have a chance to try to fill the gap somehow. While you can never replace Durant, you could sign a capable body to help somewhat fill the gap (Deng, Batum, Jeff Green, Evan Turner, Joe Johnson, etc.) or make at trade to get somebody better than Andre Roberson (assuming the Thunder could sign anybody with Westbrook/Ibaka more likely to leave the next year w/o Durant). It would also give Donavan a chance to set up a new offensive system to take advantage of the players he does have (Waiters, Kanter, Ibaka) instead of being so Durant/Westbrook focused.
I think a healthy Thunder without Durant, with a full offseason to prepare and plan, would win 45-50 games, and would still make the playoffs out West.
Let's flip it around. How many wins does just Russell Westbrook get you? I think just him alone can get most teams 40 wins.