John Lewis, the music director of the Modern Jazz Quartet, noted: “Jazz developed while the great popular music was being turned out. It was a golden age for songs. They had a classic quality in length and shape and form and flexibility of harmony. The jazz musicians were drawn to this music as a source of material.” The Songbook, a product of a fleeting set of cultural circumstances when popular, sophisticated music was aimed at musically knowledgeable adults, was the crucial wellspring of jazz. Both jazz and its progenitor are worthy of radical—indeed, reactionary—efforts to preserve them. But despite Gioia’s ardency, there is no reason to believe that jazz can be a living, evolving art form decades after its major source—and the source that linked it to the main currents of popular culture and sentiment—has dried up. Jazz, like the Songbook, is a relic—and as such, in 2012 it cannot have, as Gioia wishes for it, an “expansive and adaptive repertoire.”
What has been interesting to consider about the musical vocabulary of the 19th and 19th centuries and the 20th centuries on the one hand, and the evolution of popular music in the U.S. in the 21st is a question, a pragmatic and practical question. What we can see in music in the West is that harmony plays an unprecedented role in defining musical language, in ways that are not the case elsewhere and that aren't even the case prior to about the 1700s.
The major/minor key system we have taken for granted is only a few centuries old at most and what happened in 18th and 19th century art music that doesn't seem to happen much in contemporary popular music (perhaps?) is modal mutation, shifting from C major to C minor or from C minor to C major. When jazz musicians find contemporary popular music wanting it may be in part because once the chords for a new song are laid down there's nothing more to add than just playing the songs and if you try to play the changes the changes may not let you change mode without doing violence to the fabric of the song.