Agreed, but the changes started long before 10 years ago--when he moved to Denmark/New York (when was that--late 1980s?) and the program came out of hiatus the first time, it was already different--he was raunchier, more _obsessed_ with sex. He was probably always a bit sex-obsesssed even before New York--having an affair with his producer then abandoning her for the Danish exchange student from his high school years. But he was careful about letting it intrude into the broadcast. I noticed a ratecheting-up of the raunchiness on the program when he reintroduced it under a different name (which I've forgotten), based in New York. It was masked to some degree--was not in-your-face, but more frequent and to me, unmistakeable evidence that his mind was becoming obsessed.
It was downhill from there. I agree that the slide has become more precipitous, esp. since the late 1980s.
I have a audio cassette collection that a friend gave me of (I think) Keillor's most recent book. It was entertaining, mostly. But this current set of Lake Woebegone tales smacked of Playboys hidden in the corn crib, of dirty jokes told by adolescent boys when nobody else was around. It amused me, and left me feeling unclean afterwards, and I haven't listened to them again, unlike some of his early written work that I reread often. But like Heinlein, it stopped being about writing what his readers wanted to read, and became more and more about using his celebrity to get people to read what he wanted to write.
I fondly remember "The Golden Age" of The Show. There was a time when it literally felt like you were listening to some kind of "Midwestern Hayride" broadcast on your own little 5000 Watt station, someplace local. All the people were old friends. You didn't expect perfection, just a good time, because they were old friends...the thing you did Saturday night. But with success came power, and we know what power does. Before long, The Show came from an endless series of On The Road venues, as they sought to burn the money that was being generated for a "public radio" show. And gradually, I began to see that the Emperor had no clothes...or at least, feet of clay.
Age will make fine wine at first merely odd, later unpleasant, and finally no more than vinegar. I enjoy a little vinegar in foods sometimes, but not when I thought I was ordering fine wine.