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.)
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.)