Because he will be 31 when he can opt out, he can decide not to but that puts him at a risk of not getting as good of a contract when he is 32.
That's exactly what happens almost everytime a star can opt out. They do, you try to sign a new contract as soon as possible again instead of putting yourself into a situation where you could end up being injured and out of the league with no more contract.
If he can get 5 years/50M$ after next season he would be smart to opt out of the final 16M$ year because he might not get 5yrs/50M$ at 32 again.
This is a situation like Lewis will face this summer.
Right now draftexpress if projecting the Blazers taking Durant which would leave Oden to the Sonics. This makes sense on a need basis, but if the Blazers decide to unload some of their guys and build around Oden; then Durant would go to the Sonics which would make Lewis redundant.
Lewis should be a hot commodity, but with only a few teams under the cap I'd say Orlando is the only team that would likely to be bidding big money.