I was thinking, however, more in terms of the kinds of things David Weber cites. A dam engineer, for example, used to have to know structures, materials, hydraulics, hydrology, and geotechnics. Today, that same engineer is also expected to know about ecosystems, biology, policy and public relations, etc. The control volume has become far broader than the reservoir and the dam and now extends up and downstream. This takes time and energy that once might have gone into learning other languages, studying politics, or whatever. As you see with your wife who comes home and just wants to unwind watching mindless TV by the end of the day, thinking requires a lot of energy, which tends to be, at some point, finite. (Medicine and vet. science have also become much more involved in the last fifty years.)
On that track, I will throw out another idea. The prevalence of mindless TV is not the cause of our loss of cleverness, it is but a symptom.