Disagree with the bolded. OKC could trade Harden for a lottery pick and an impact player still on a manageable rookie deal. They could actually save a good bit of money, avoid the tax, and still get good value for the current roster while pocketing a lottery pick for the future. If OKC flipped Harden for, say, Brad Beal and a future top five pick from Washington, or, say, Gerald Henderson and an unprotected lottery picks from the Bobcats, that's acceptable quality at roughly a third of the salary, plus a high-end draft pick who'll play cheap for 3-4 years. That's what I would do.
As things stand, OKC isn't seeing the top of the draft for years without making a trade. Harden, as was ably pointed out above, is likely to be simply too expensive for the Thunder to retain, regardless of whether they "want" to lose him (see: Asik, Lin). But if OKC works proactively, they could use Harden as the chip that replenishes their draft pipeline while not losing too much quality in the near term. Those proposed trades are just hypotheticals, but you see my point -- OKC can move Harden, avoid the tax, still get an NBA starter on a cheaper deal, all while planning for the future.
I wouldn't cut Perkins at this point, even though it likely costs you Harden (through a trade). They need someone to guard Howard, and they aren't going to find anyone on the scrap heap who can do it as well as Perkins. Center just became a whole lot more important as a position in the West. OKC will need Perkins in the WCFs.