What makes avant-garde avant-garde?

[Vincent A. Maeder]

Otto wrote "It's difficult to think of any artistic avant-garde these
days."
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I agree, which is what prompted my initial question.  My fear is that
over-commercialization of the art process and the co-optation of
potentially vanguard movements (especially in music) that very little is
produced which could be regarded as revolutionary.  Presumably, the
adaptation of the title "avant-garde" to any author/artist/musician is
the kiss of death or perhaps the tolling of the end of the revolution.