Ben Yagoda's perceptive book The B-Side (Riverhead) laments the passing of the era of the "Great American Songbook," a period that lasted from 1925 to 1950. While his bemoaning of the end of the "standards" period is insightful—rock 'n' roll was not to blame, he argues convincingly—he also delivers sideways insight into the constant evolution of pop economics as well as pop aesthetics.
In 2013, 1.26 billion digital music tracks were sold. In 1917, over 2 billion songs' worth of sheet music were sold. Foreshadowing the arguments over music pirating in the 1990s, unauthorized performers of popular songs in the early 20th century argued that they were doing the copyright holders a favor: Their public performances helped to boost sheet music sales.
A giant industry built on selling recorded sound may prove to be a historical blip, but new ways to get people to pay arise unexpectedly and it's a good bet that people will continue making and loving music.