It is like what happens often in baseball where Team A trades a prospect to Team B for say a closer. The closer helps Team A make the WS but then flames out and the prospect becomes and all star in 5 years. Who won the trade? Do you know for sure Team A would not have made the WS without the trade? No but that would generally be the conclusion and criteria for assessing Team A's success. Team B doesn't even think about judging the trade for years.
So you judge the trade based on what the team got out of it relative to what they hoped for. If the closer pitched well and saved some big games, it is hard to say it was a bad trade for them even if the prospect becomes Albert Pujols. But then 5 years later all you hear is, "can you believe that Team A traded Albert Pujols"!
It is funny that the Celtics, a title contender, did kind of the opposite. They gave up a proven starting center (albeit injured at the time and with uncertain health moving forward) for a lesser center, a young player at a position of need, and a future draft pick. So do you use the short term or the long term criteria? Say the Celtics flame out in the playoffs, the Thunder make the finals, and the future pick is another Paul Pierce. Who wins?
Most fans are going to want titles but maybe the owners want long term return on investment. I suspect the spirited debate is going to continue on this one for years.