I think the OP was referring more to educational material on channels like Discovery and History giving way to fiction based shows. Not that fiction based shows are "getting dumber". That's a different topic.
Yep. I remember when A&E and History Channel had a lot great stuff that actually had history documentaries. No offense to anyone that likes Pawn Stars, Auction shows and Pickers et al..
And MTV doesn't show as many music videos anymore. And Syfy doesn't just show reruns of old space operas and superhero shows anymore.
This may have less to do with the dumbing down of the audience and the discovery that cable channels that are too
specialized don't attract enough viewers--and have to broaden their programming to survive.
(I remember when people used to complain that the History Channel ran way too much material on World War II, as opposed to the rest of History!)