I've read a bunch of these types of takes (though this is definitely one of the better referenced & more comprehensive ones) & there's an aspect I haven't seen raised in the context of the correlation of modern trends/changes with declines in literacy, IQ, etc.

The immediate goto is always either smartphones (as in this piece) or the world wide web. Other pieces looking at longer term trends might even reference TV (the dawn of media entertainment consumption without intentionality).

These are all focused on leisure: what we do in our spare time, presumably because we mostly read for pleasure in our spare time. What I rarely see highlighted is the growth of the so-called "knowledge economy" & the shift of common "workers" being employed in physically skilled jobs to common workers being employed in sedentary jobs requiring the application of (basic) literacy for most of our days.

Given we spend a lot more time in work than we do at leisure activities this contrast of an increase in (relatively mindless, unstimulating) "literate" activity in work with the decline in (focused, stimulating) literature-oriented leisure activities seems relevant in my mind.