I don't buy it completely. There is a real chance that Duncan hangs it up after this year, and if so San Antonio will be a worse team.
Harrison Barnes might also leave GSW, and while that isn't a huge loss, it does weaken them.
Durant and Westbrook on OKC are a top 4 team in the NBA. If the two scenarios above happen, they could be top 2. Where does Durant go, besides GSW, to put him in a better situation than that? There isn't another team that offers more than his current squad because they have Westbrook.
The only reason I do buy it is if Durant believes that he can't get there with Westbrook (or with Billy Donovan as coach -- more likely). If either are true than it makes sense to leave, but again I really don't but that he will have a better chance anywhere else than OKC.