I'd also like to add that Ibaka and Westbrook are off contract in 2017 so that gives them the option of making an ultimatum to OKC management, either add to the team and max out the cap or we all walk.
Blake Griffin, Chris Paul, Gordon Hayward and Paul Millsap, are all free agents in 2017 too.
Ibaka
Griffin
Durant
Roberson
Westbrook
1) They've been in the luxury tax the past couple years, significantly so this year.
2) If KD resigns a 1-year deal this summer, the Thunder will have no cap room in 2017 with just three players under contract (Kanter, Singler, and Payne), and the cap holds of Durant, Westbrook, and Ibaka.
I do think Durant will sign a 1+1. I've thought that was most likely thing for awhile, and even moreso now. But without some kind of major trade, that team is going to have the same personnel, give or take some MLE and vet minimum types, kid of like the Celtics beginning in the summer of 2008. They had their team, and they could try to improve it around the periphery or hope a draftee stepped up, but the options were limited. Durant, Westbrook, and Ibaka can decide together if that's what they want, or if forging a new trail makes more sense.