Sexton also wanted a max extension last summer. He may have realized he's not getting max money at this point but he'll want close to max money despite having virtually no suitors.
If he were to change his game and become a modern day Lou Williams, Jamal Crawford sort of microwave off the bench then he'd have the opportunity to be an impactful player on a contender but players don't usually adapt like that so early in their career on their own. That's his best bet at having a long and fruitful career though.