Brooks has consistently been one of the five worst coaches in the league throughout his OKC tenure, so you have to pin a significant amount of blame on him.
On the other hand, I don't know how you can't ultimately say the Harden trade
wasn't the main reason they failed. It's virtually impossible to organically assemble a team with three of the best 10 and four of the best 25 players in the league in the salary cap era--you have to draft perfectly, which they did, let alone keep them together past their rookie contracts. They were also gifted this opportunity by the league with the duplicate 5-year extensions for Russ and KD, and Harden willing to forgo that 5th year and sign for 4/$60 or so. And they blow it up over... what? A few mil a year?
You simply
cannot put the financial hit of the luxury tax/wasted Perkins amnesty money ahead of a literally once-in-a-generation collection of talent. It's nothing short of a disgrace to the NBA, to the game of basketball, and everything that sports should represent and stand for.

Frankly, they deserve to lose Russ and KD if you ask me. They did them dirty.