He's got a Players Option next season, which I'm sure was built into his contract for good reason. Duncan gave himself that safety valve, in case the wheels were starting to fall off. Honestly, the way the rest of this season plays out will likely dictate whether Duncan opts out or not next year.
If the Spurs lose in heartbreaking fashion again, will it break his resolve? Or will it fuel him to come back and give it one more shot? From what we've seen from Duncan historically, I'd be inclined to believe the latter.
If the Spurs get their asses handed to them somehow, Duncan is probably gone.
If the Spurs win the championship, will Duncan want to come back for the repeat he never got, or will he follow DRob's footsteps and retire into the golden sunset? Seems like a coin flip to me, tbh.