In the notes for Bill Evans: The Secret Sessions, I observed of certain Free Jazz or New Thing players,
The movement did attract a fair number of poseurs enchanted by the idea of playing music without having to know anything about it. Today, most of them are otherwise employed.
At least, they had instruments.
The blogger known as Shrinkucci, who is a drummer and a psychologist, posts an interesting story about a young man who, because he wants to be, believes himself to be a great drummer. To read it, go here.
While I understand your point here, all but many, many musicians (as well as composers, writers, dancers, painters, etc) who were enchanted by the idea of making art & who really DID know something about it, are also otherwise employed. Becoming a working artist in any genre is extremely difficult. Most people who try to make it end up doing something else for a living. Fewer than 5% of people who graduate with MFAs end up working as creative artists ten years later.
There are so many factors involved in addition to talent, skill and training, and there have always been so many more artists-in-training interested in careers in more traditional, less avant garde, styles of art, that I would be quite surprised if there really were a significantly larger proportion of avant garde musicians (schooled or unschooled) who were not currently active compared to the number of mainstream artists who likewise have fallen by the wayside.
Most people who want to become artists don’t. The market is simply too small.