3. And if they do read something, they don't understand what they read, either because it's poorly written or because they start from different assumptions than the ones the writer had in mind.
I like to think about design as a communication medium.
So for most stuff the manual goes straight to the shelf. Unless it is something particularly powerful or complex, I see having to refer to the manual as a failure in UX design.
Is there any point in tattooing it on the back of the hands of people who don't read?
That said, https://readabilityformulas.com/readability-scoring-system.p... is a good sanity check. Over half of people can't read text above a grade 8 readability. Worse yet, we aren't conscious of the effort that reading takes. Not until we are struggling. So competent people have little sense of the barriers they create.
Of course, they won't read it before they design something that requires deep engagement through reading.