I will agree that TV has to a great extent NOT been as educational as it could be. Consider, prior to 1938 the average education level in the US was about the 7th grade, people graduated at age 16 and either wound up in the 'trades' or on the farm, only a few recieved an adequate, or better yet superior education. WW II helped increase the access to education and new technologies along with developing theories of learning opened new vistas of data and learning.
In other words I think it is not dumbing down but excess knowledge and inadequate methods and means of teaching. The idea that we here in the US have 'failed' to teach is ... IMHO ... a misinterpretation of lazy thinking.