I couldn't help but think that TFaS was the product not so much of a desire to share some new insights he'd recently discovered but rather that he saw (or some publisher pointed out to him) how authors such as Pinker, Greene, Tyson, et al were garnering considerable attention and revenue translating sophisticated scientific topics developed by others for a lay audience. As if someone said to him: "Hey, you were involved in discovering some sophisticated and important topics, why don't you translate those to a lay audience?!"
I have great respect for Kahneman and H&B is a wonderful collection. However I find it peculiar that the entirety of TFaS seems, on the surface (again I've not read it) strikingly similar to essay #22 in H&B by Steven A. Sloman titled "Two Systems of Reasoning". In it he uses Kahneman's and Tversky's work to argue for two competing strategies for arriving at the truth: associative vs rule-based.
It's as if Kahneman is borrowing from Sloman who borrowed from Kahneman.
It's first and foremost a heuristic -- a reasonably good way to generate good behavior. Secondarily, in certain specific situations, it causes non-optimal behavior.
 ( https://www.amazon.com/Heuristics-Biases-Psychology-Intuitiv... )