There may be correlation, but what about a larger context, say go back 20 years or so?

Look at other segments too, for instance recipe books (or even websites). Totally worse in any way than using an LLM to make a recipe with only the ingredients you have on hand.

And, is it really causation? Could it be that more and more people just pirate the book from libgen ?

Could it be that people don't read much anymore and look for self-help on youtube or tik-tok?

The hypothesis is interesting, but it's hard to prove.