I personally don't think there's any real blame on anyone.

If anything Presti failed by not convincing the owner to pay the tax and keep all three. But he couldn't have forseen that Harden would turn into an MVP candidate and might have been the player to keep instead of Durant or that Durant and Westbrook's relationship would turn sour enough to keep Durant from staying.

Zach Lowe talked about this on his podcast, but because of some loophole in the salary cap, Durant's contract was given a type of classification that took the Thunder into the tax. But after some digging, the Thunder found out that the NBA made a mistake and that Durant's contract was misinterpreted. By the time the error was found, Harden was traded.