Some team will offer him the max or close to it and the Thunder won't be able to keep him, what with the luxury tax.
If they were the Lakers, sure, they'd keep him, go way over the cap, and pay the tax. The Thunder? I don't think so.
I really don't see how they could possibly keep Harden.
BTW, he essentially is starting now. The fact that he "comes off the bench" is a ceremonial thing. Look at the minutes he plays. He's a starter. Same for Manu Ginobili.