The last of the four podcasts that David Hiscox, Mark Moncrieff (of Upon Hope) and I recorded was on the topic of the decline of classical music. David is especially knowledgeable in the field and explains clearly why modernist classical music often sounds cacophonous. You can listen to the podcast here (it runs for 30 minutes).
I should add that since the 1990s (roughly speaking) some composers have begun again to compose music that is more likely to appeal to classical music fans and more likely to win a place in the canon. As an example there is the piece below by the English/German composer Max Richter.