Earlier quoted context omitted.
Philip Glass is constantly described as avant-garde. You may take issue with that label, but I hardly see how it can be surprising.
It's a ridiculous label to apply to Philip Glass. He would likely reject it himself. The word "avant-garde" comes up not once in Philip Glass's Wikipedia article [0]. The only uses of the word "experimental" are in reference to his early interest in experimental theater. Philip Glass is easily the most popular, mainstream living music composer. His music is extremely "listenable," and inoffensive to the casual listen…
Clearly looking at his age he is already an elder. Nevertheless he definitely qualifies as avant- Garde. You can compose listenable pieces and be avant- garde