I have way less knowledge than some of you on this, but I think the circumstances more than the music has changed or died. Part of the reason there is an avant-garde, I think, is the conservative nature of music from early last century. People said 'fuck it', and tread their own paths, often shocking and pissing off the conservatives along the way.
Now we have the strange situation of anything shocking or strange being either deemed 'cool' or edgy', or simply derided as being too safe etc.
Didn't many of the early avant-garde artists start in the system, making it easier to fuck with it? I'm not massively knowledgable here, but that's the idea I always had of this genre.
Everyone has individual ears, so one note to me is another to you, but maybe the spirit of a true 'fuck it' mentality has eroded into an 'I'll be cool now' one...
I'm probably rambling, so now I'll leave

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