People performed and wrote popular music using much the same language as classical musicians. The concert world played major league franchise to popular music's farm team. Roughnesses and banalities from below were weeded out. But American classical music is not necessarily a destination anymore. From one end, Europe still plays mind games with classical music's head, and from the other, the immense force of music from Africa has cut it off at the legs.
Popular music has a separate language now, with many fewer words in common.
Even the techniques are different. Two worlds once interdependent are not so happy in each other's company anymore. Gann is understandably fearful of European cultural oppression and the inferiority complexes it has caused. His fears are justified by the vast mediocrity most American concert composers continue to offer. It is a mediocrity supported not by Americans in general but by philanthropic cultural minority, and that without much enthusiasm.