I don't think OKC regret the trade, they have done well without Harden. No discussion that Harden is the superior player but OKC got a perfect 6th man who doesn't need the ball in his hands basically giving Durant and Westbrook more of the ball. Signing Ibaka was the right decision because he provides defence same with Perkins. You can't pay max and use him as a 6th man. OKC has no problem scoring so they had to shore up defence, and Martin has done a great job.
This is one of the rare cases where both teams benefited. We got a superstar they got cap space and a young player and a draft pick.
While OKC made out okay in the deal, I am pretty sure that it was made for financial reasons. I am almost entirely sure haha. If money were not the issue, Harden would not be moved, so in no way did OKC "get better" moving Harden. They just more or less had to, or had to make some move w/ some player.
Signing Ibaka was definitely the right thing to do, but again, OKC traded a greater chance of winning now (say this season and the next couple) for being "in contention" many more yrs down the road. A nice draft pick, a nice young prospect...they're shoring up for the long haul.
From an owner's point of view, in a smaller market, this is the only move that makes sense. Winning a championship might not bring much more $ to the bottomline, but competing for one for many more yrs, even if you have a lower chance of obtaining it each attempt, could net you a lot more $ when all is said and done. OKC fans already always coming out to support the team, so being in contention is enough. If OKC's owner cared more about winning now (this yr to next 2 yrs), he would have just paid a bit more cash out of pocket and had a greater chance at a title. He didn't because he apparently values x dollars over y yrs of contention more than the z dollars and v intrinsic value he gains from winning a championship. That's fine; worked out for us!