It's quite simple really. He stands to make more money now if he can renegotiate his contract, than he would make waiting for next year's free-agency.
It's the same strategy the Celtics were trying to use if we got him here (if we were indeed interested in him).
OKC has cap space to burn, so if Westbrook wants to optimize his money making ability, he'll renegotiate now.
So, yes, that would make sense financially for him to renegotiate his contract to essentially earn the max this coming year at $26.5M. But if he did the rumored three year renegotiation/extension deal, wouldn't that actually cost him money in 2017-2018 by doing that rather than a 1 + 1 next summer?
Essentially, he'd make $8.7M more this year with the renegotiation and extension. However, that'd put him at $28.5M in 2017-2018 rather than $31.2M if he'd do the 1 + 1 next summer, and that's going with the lowball estimation of the cap at $104M. So he'd ultimately gain $6M even by renegotiating and extending the contract rather than doing a 1 + 1 next year, but that comes at the cost of not having the free agency option this summer, too.
So the question is this - is an extra $6M worth losing the free agency option to explore differing teams this summer and commit the next two years to OKC? I don't think it's worth it for Westbrook if he's really wanting to compete now, which OKC just can't do.
I'm fairly sure the current estimate of the max salary for Westbrook next year is $28.9 million, based on a $102 million salary cap. I don't remember the number off the top of my head, but it was between $28-29 million, per an Eric Pincus tweet last week (he tweets way too much to find it exactly.) So based on the current salary estimate, he'd come in right around even for next year, while picking up an extra almost $9 million this year. Furthermore, he gets more money upfront (so add in a little bit extra for the time value of money).
He does forego a year of free agency, for $9 million. But it also lets him leave without being the bad guy. He can have given it two years without KD. I also imagine he'll have a trade bonus in there so that he'll be able to get a little bit extra if OKC trades him next summer to LA.
In other words, I think he's going to have his cake and eat it too. Extra money this year, no loss of money next year, and able to sign a contract with full Bird rights at 10 years of experience with the team of his choosing, be it OKC or whomever he works a trade to next summer.