The emphasis here is on worthwhile: the idea that changing your mind, and knowing how to, has a tangible benefit, and one that is (generally, on average) worth the effort it takes to learn. If there's no particular benefit to changing your mind, then either (a) you have already selected the best outcome or (b) your choices are irrelevant.
If this is the best possible world, then I feel okay calling us doomed; it's a pretty lousy world.
As to irrelevancy, well, to think that I'd live the same life regardless of whether "Will you marry me?" is met with yes or no? That is not a world I want. The idea that given a set of choices, the outcome remains the same across them is just a terrifying nihilistic idea to me.
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.
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.