Not quite sure why everyone thinks OKC took a step back. That team is still LOADED with talent. Kevin Martin is no scrub, he can score and should thrive with defenses focusing in on Durant and Westbrook. He's not the facilitator that Harden is, but they're getting Eric Maynor back this year will alleviate that with the 2nd unit.
If the Spurs traded Manu near his peak as a player, would that have set them back? Because that is what OKC just did.
There are two ways of looking at this trade.
First, the reality: OKC was probably never going to resign Harden long term. They could have played this year out, go for another title run, and then dealt Harden next summer. That is like knowing your marriage isn't going to survive, but putting off divorce for one more year just so you can save more money and sell your house at a higher price. There is no guarantee that outcome will happen, just like there is no guarantee that OKC would get back to the Finals. The fact is that OKC is selling Harden high. I don't disagree with making the move now, but OKC in the near term is a worse team for it.
Second: OKC also is clearly assembling pieces to make another major move to add to Durant + Westbrook. You don't resign Ibaka and establish your young core, then trade away one of the biggest pieces to that core for a nice veteran swingman (Martin), an unproven mid-tier rookie (Lamb), and draft picks. Those of you on this board who are arguing that Martin replaces Harden are insane. Harden was a former Top-5 pick, 6th man of the year, and a member of our Olympic squad (you know, the team MVP Rondo
didn't make!) Harden, quite simply, is one of the best young veteran basketball players ON THE PLANET. Kevin Martin cannot hold his underwear. Thus, it is clear to me that OKC is assembling pieces to add another top-level NBA player to Durant+Westbrook+Ibaka, while in the mean time trying to compete with Martin as a decent filler.
All in all - OKC is not a finished product. And it is sad that their Big Three had to be dismantled because Presti go hoodwinked by Danny Ainge in the Perkins trade.