I accepted this paradigm for a long time and only much later did I really started to doubt it. I think the first inkling of a problem came in undergraduate music when I was in the listening library one day sampling some new music recordings. I happened to put on a disc of some Stockhausen for multiple orchestras (Carré or something) and followed it with Drumming by Steve Reich. The Reich was much more interesting and made a larger impact for me because it was not maximalizing dissonance and complexity! Well, according to the aesthetic parameters that I had learned regarding 20th century music, that was just wrong!
Technical progress in writing music is a very problematical concept as soon as you start to unpack it. For one thing it tends to place music in the same realm as science: research and development of new musical ideas. Milton Babbitt is the locus classicus for this viewpoint; another prominent practitioner is Pierre Boulez. Let's listen to some Babbitt. Here is his Composition for Four Instruments from 1948:
How are you supposed to listen to that? Are you supposed to hear the structure? Do you have to listen with the score? Should you study the score first? What kind of aesthetic experience could you have? Also, I have to point out that, there are a distressingly large number of pieces in this genre that all sound the same to me: the disjointed, fragmentary rhythms, the wide leaps in the melody, the huge changes in dynamics, the chopped off phrases--it all adds up to the same aesthetic effect. For me.
This is music that seems to be focussed only on the process of creating it. The syntax of it. The aesthetic effect is somewhat secondary. But it wasn't always so. If we go back to Schoenberg, Berg and Webern, two of the three always seemed to have also have a semantic or content side as well. In his early works such as Verklärte Nacht or Pierrot Lunaire or even the Six Little Piano Pieces, op 19, Schoenberg has a great deal of 'content'. Later on, he seems to have less and less, though there are exceptions such as his unfinished opera Moses und Aron. Some of his content seems to be related to issues surrounding his Jewish identity and the rise of anti-semitism in Nazi Germany. In other words, extra-musical events.
Schoenberg's student Alban Berg seems to have even more leaned towards the 'content' aspect and away from the structural aspect. His Lyric Suite for string quartet, long thought to be simply abstract music, turned out to have a rather detailed secret program referring to an affair between Berg and Hanna Fuchs-Robettin. Here is the first movement to give you a sense of the music:
While written using 12-tone procedures, that music certainly has an expressive 'content' in the sense I am using the word. Berg, as the author of two successful operas, Wozzeck and Lulu, has a strong sense of theater in his music. This has not ever been discussed to my knowledge, but his Violin Concerto contains instructions to the orchestra to perform certain actions onstage--again, the result is a kind of theatre. Mind you, I have not noticed any orchestras actually enacting these instructions! But they are there.
Anton Webern went in quite the opposite direction: he has the least 'content' of the three and his music seems to be the most concerned solely with the technical aspects of composition. It is hard to imagine a 'secret program' in any of Webern's pieces! Here is his String Quartet, Op 28:
I think you can hear how that is going to lead directly to Babbitt and Boulez. After the war the choice was made in places where composition was being taught, like Darmstadt, to follow the example of Webern and not the example of Berg. We might speculate that with the just past horrors of the war, the last thing anyone wanted was aesthetic content! On the other hand, this didn't seem to stop people like Shostakovich from writing content-full music after the war. But in Western Europe and North America, the ideology of music composition was to take Webern as a model and stress the syntax of music while ignoring the semantic. Even though this model has begun to break down there are still places that turn out composition students who seem to follow it.
If I might inflict on you a metaphorical way to distinguish Schoenberg from his students I could compare Schoenberg himself to a hearty dish like wienerschnitzel which would make Alban Berg a Sachertorte and Anton Webern, well, schnapps!