A simpler explanation is simply the changing market and distribution. It was really only in the last two hundred years that composers could support themselves by selling their music rather than relying on patronage, with Beethoven being the first to do so. Cheap recordings and broadcasting have changed the game entirely. You might not be able to name a single popular singer of the 19th century, but that's not because they didn't exist; the practice goes back to the troubadours of nearly a thousand years ago.
It's not that art music has had a big fall from grace and lost its grip on the public's heart, paving the way for rock and pop. It never had that grip in the first place. The musical descendants of Bach and Beethoven are alive and kicking; just look at John Williams.
Are you comparing John Williams to Bach and Beethoven? Williams is a very skilled film composer who's great at borrowing ideas from better composers and reshaping them for film. But his music is very unoriginal on its own.
His point was that there is still complex music being composed in the "classical" tradition, outside of pop, regardless of its actual merits or popularity.
Yes and no. It was easy for Bach and Beethoven to be innovative, less had been done back then. The story of classical music from 1800 to 1952, though, is a story of composers forever looking for new ways to innovate. First they did away with all the old rules of structure, and we didn't miss them too much. There's some great music from this period.
Then, they started doing away with all the rules of harmony. The prelude of Wagner's Tristan und Isolde has a dissonant chord which resolves to another dissonant chord, and it totally blew everybody's mind. By 1913 you've got The Rite Of Spring which avoids almost all traditional harmonies, and it's causing riots in Paris but a few years later that kind of thing is old hat. In the 1920s you've got Schoenberg going "Y'know what? Screw tonality. Let's just play all the notes one by one; at this point you've got music which very, very few people out there can actually enjoy. I mean, I have a CD of Schoenberg's violin concerto, and even the blurb on the back of the CD doesn't seem to like it very much.
Then you've got a few decades of increasingly obscure music until my chosen endpoint of 1952, which is the year when Johnny Cage wins a flawless victory in the "look how unmusical I can make my music" championship by publishing 4'33", the piece consisting entirely of silence. This was the signal that the old musical tradition had finally disappeared up its own butthole. (1952 is also the year when Rock Around The Clock was first written.)
Anyway, John Williams is basically an acknowledgement that if you're going to make big orchestral music which people actually like, then all the good ideas were taken by 150 years ago, and any additional innovation away from that point pretty much just makes it worse [or, at least, less enjoyable by the majority of people].
Oh, it's definitely true that many composers reacted and "innovated" away from harmony, and that rock was developed around the same time. But there was a great proliferation in styles in the beginning of the 20th century. During that same period, you have guys like Rachmaninoff, Holst, and Orff -- different points in the musical space, and still popular today. And then you have Ralph Vaughan Williams, who went back to the time before harmony by creating new modal music.
Popular music and art music have coexisted for a long time. There are a lot of trends happening at once in music, and you've hardly made a case that Schoenberg and Cage are strongly connected to the rise of rock. I don't see why your reasoning can't be used to argue that the overwhelming complexity of the Romantics destroyed art music and handed control over to Jazz.
(And, no, John Williams is just an acknowledgement that anyone I call a descendant of Beethoven is going to sound a bit like Beethoven. I can think of several names innovating in the big orchestral space, all of which sound quite a bit different, and most of which you've never heard of.)
Definitely. But a major trend in art music relatively recently has been the mix of foreign and other forms with Western styles (and certainly true of the music I'm familiar with). I could hardly call Karl Jenkins or Tan Dun musical descendants of Beethoven. Perhaps I picked a name based on the wrong criteria.
It's not that art music has had a big fall from grace and lost its grip on the public's heart, paving the way for rock and pop. It never had that grip in the first place. The musical descendants of Bach and Beethoven are alive and kicking; just look at John Williams.