@luckytran Some of the highlighted bit is spookily on the nose, but Dumb and Dumber is harmless fun.
I'm not actually convinced people are getting dumber, and I certainly wouldn't make that case based on popular entertainment (I am a huge silent film fan, but I'm not going to claim that slapstick is intellectually challenging! And c'mon, Vaudeville?)
People have always been dumb and manipulable. The main thing that has changed is that, in previous eras, such as "the war on communism", people were manipulated via fear etc in ways that at least encouraged unity, instead of demonizing "an enemy within".
Sagan's point about the loss of manufacturing is interesting, and I think he's on to something there, but he doesn't elaborate.