I think 95% of the reason Durant left was Westbrook's attitude. You watch Durant's MVP acceptance speech and he basically says as clearly as you can say it without saying it that he cannot stand being teammates with Westbrook. And then when he leaves for Golden State, he doesn't loop in Westbrook at all. And then after all that was done, the sniping all year long between the two shows there's a lot of animosity there.
I believe when all is said and done and when people talk candidly about Westbrook it will be stated that he was a very difficult teammate.
I don't think so either, I just think he's impossible to play with. Abrasive personality off the court, and the kind of guy that is best suited for hero ball on it.
Harden was traded because playing him and Westbrook side-by-side did not appear to be optimal, and Harden wanted more usage to reach his potential.
Durant you can put on every team in the league and they instantly get better and he will find a way to fit in. He's a superb off-ball player. But he couldn't stand Westbrook, just could not stand him as a personality.
Ideally, Durant-type players is exactly who Westbrook needs. Guys who don't need the ball in their hands all the time. You could assemble a super team with Westbrook if you gave him the '09 Lakers treatment... two good bigs, a complimentary guard that is a great shooter, and versatile players at forward that can catch-and-shoot. That's basically how they've tried to configure their current team but none of the pieces are good enough. Kanter/Adams aren't elite, Oladipo is very good but not an All-Star. Their forwards are generally one dimensional and also not elite. They need an All-Star caliber compliment for him that isn't a high usage.
For all the talk Sam Presti got about being a genius, I think a lot of it is unwarranted. True, he drafted Durant, Westbrook, Ibaka, Harden, Maynor, and Jackson, but he also hired a bad head coach and none of those players actually fit together. They were more like an all-star roster than a true, cohesive team. Durant, Westbrook, and Harden HAD to have the ball to be effective (less so with Durant, but he still needed it to dominate, especially in his early years). None of them were particularly great outside shooters (Durant was best, but not an automatic guy from distance or anything like that) either. Perkins was literally a non-factor offensively, a liability. Ibaka was pretty good from 10-15 feet, but couldn't post up. Thabo was the only guy that really filled a role-- 3 and D. I guess Fisher did as well during his time there, but really brought nothing from an offensive standpoint. Collison gave good energy and hustle, but was only an adequate rebounder and offered almost nothing offensively. I guess my point is that despite having all the talent in the world, those players never really fit together, offensively. They didn't and couldn't compliment each other. Durant was the alpha player, but he didn't have the alpha personality. All the talk of him being "soft" didn't come from nowhere. Westbrook had the alpha personality, but wasn't the alpha player. I can see why both Durant and RW have both had the years they have had this year.
I believe you're right in pointing out that OKC does seem to be more building a complimentary team today. In those days, I think they were doomed from the get-go though. I blame the make-up and chemistry of the team (management), as well as the cap/economic situation/OKC market of the time, to a lesser degree, for the Thunder's failures. Not Westbrook.