They barely received anything in return from Houston (think about it; that's a 20-25 win team before Harden got there).
a) Harden is not worth 25 wins on any team. The Thunder went from a 71% win team with him (41 wins/19 losses) to a 73% win team without him (60 wins/22 losses).
b) They received a near equal scorer in Martin (for Thunder's role); Lamb (who has a high ceiling and should already be in the rotation); a 1st round pick in the 2013 draft from picks 4-14 and picks 3-14 in the 2014 draft (a great asset to draft a big or to use in a package deal to net a big):
A pick that could net them or a team - Cody Zeller, Alex Len, Trey Burke, Anthony Bennett, Marcus Smart, Julius Randle, James Young, James Harrison or Willie Cauley-Stein etc etc.
Plus another 1st round pick and a second round pick. That right there is more than an adequate enough value for Harden (himself) and for what he was gonna produce for the Thunder. He was never gonna provide for them 20+ points and the
impact he has for the Rockets, there isn't enough wing production to go around between the three of them to warrant all max offers.
All they had to do in the offseason was match whatever contract Harden was offered (would have been four years, not five) and if they didn't want to keep him long term, trade him after December.
The Thunder were only ever gonna offer him $52 million, there would've been countless amounts of teams that would've offered him more than that. They were never ever going to exceed that amount to match any offers in free agency. The Rockets signing him to an $80 million deal only foretold the situation of what teams would've offered Harden had the Thunder kept him.
You think Minnesota wouldn't deal Love for him when it's doubtful he'll resign with them? There would have been plenty of teams that still would have traded for him
Why would OKC trade for Love when they've already got Ibaka? Plenty of teams would've traded for Harden, but there's a miniscule chance OKC would've got back the amount they received from Houston. As I said before, teams would've bargained OKC down as much as they could; Thunder would no longer be in the driver's seat in a trade sense, had they kept Harden for the season.