What he found was that as cable marched across Norway intelligence fell in its wake: “for each year that their hometown had full coverage, IQ dropped by 0.08 points”. “Higher exposure to commercial television”, Hernæs’s study bluntly concludes, “reduces cognitive ability”No doubt that effect is real. But I think of it as this: some activities excercise your brain in ways that improve it, make new connections between neurons etc. While other activivities don't. Or worse: degrade your brain. Aaand: which is which, also depends on what you've done before. Or how often you do it. Or in what environment.
People who read books all day, every day, may become walking libraries who know all their classics. Are they smarter? Have more 'agile' brains? I doubt it. Perhaps the brain specializes on that activity & becomes veeery good at it. Potentially becoming worse at other activities. Granted: there's probably a lot of 'unused' capacity in most people's brains such that improving one area doesn't come at the cost of other abilities.
But I do suspect there's the rub: a healthy brain must be excercised regularly. Not just by doing again & again something it's already good at (however challenging). But also by trying new stuff. Activities not done before. Explore new subjects in entirely different fields. Especially stuff that promotes imagination / creativity as opposed to more passive consumption like movies or music. Couch potato? Move your muscles!
I'm not sure whether there's a way to max out that brain excercise part (if healty, and assuming it's possible in the 1st place). But chances are reading would be part of it. Longreads & a broad range of fiction in particular. Some puzzle or exploration / building type games could be. Mechanical construction stuff a la Lego surely is. As well as a variety of sports (doing, that is! Not watching or discussing). Socializing in various ways: in moderation.
As for people choosing (?) to spend a big chunk of their lives on passive consumption (great Dutch wording: "je leven wegflixen"): let 'em. If post-scarcity, information-overloaded society moves the masses' lives there, so be it.
Personally I prefer a hike up a mountain, improving a hands-on creative skill, a thoughful discussion, or a deep dive into an interesting new subject.