The claim is that, for lots of people, the net gain from changing their mind is so minimal as to not be worth the time spent studying. This implies strongly that, for lots of people, they have either (a) already made the best choice or (b) are not faced with any meaningful choices.
(a) implies that either lots of people are completely incapable of good decisions or are the Chosen Of God, their every selection Divinely Inspired from amongst the best of all possible worlds. Which goes back to this being a pretty lousy world.
(b) flies in the face of all the major decisions people normally make (marriage, buying a house, having children, etc.), and suggests that, statistically, a lot of the "important decisions" in my own life are probably meaningless unless I am the Chosen Of Bayes, specially exempt from the nihilism that blights the mundane masses.
For some people there may be the class (c) that the cost of learning rationality is much, much higher than normal. If your focus is on this group, that's a whole different conversation about why I think this is really rare :)
Just to begin with, the above is a terrible way to structure an inductive argument about something as variable has human behavior. Obviously few people are "completely incapable of good decisions or are the Chosen Of God" and no important decisions in life are "meaningless". It is, however, the case that most decisions don't matter all that much and that, when they do, people usual do a pretty good job without special training.
But the real issue that you're missing is opportunity cost. Lots of people don't know how to read or do arithm...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
If continuing the discussion becomes impractical, that means you win at open threads; a celebratory top-level post on the topic is traditional.