Basically, the problem is that K&T-style insights about cognitive biases -- and, by extension, the whole OB/LW folklore that has arisen around them -- are useless for pretty much any question of practical importance.

I agree for most topics, but there are applied cases of clear importance. Investment behavior provides particularly concrete and rich examples, which are a major focus for the K&T school, and "libertarian paternalists" inspired by them: index funds as preferable to overconfident trading by investors, setting defaults of employee investment plans to "save and invest" rather than "nothing," and so forth. Now, you can get these insights packaged with financial advice in books and the like, and I think that tends to be more useful than general study of biases, but the insights are nonetheless important to the tune of tens or hundreds of thousands of dollars over a lifetime.