The claim isn't that it isn't worthwhile to learn rationalism, period. The claim is that for lots of people, it isn't worthwhile.
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 ma...
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.