While the researchers in TFA took account of "harmonic" and "timbral" chnges, whatever that means, the study is still meaningless because it doesn't take into account:
1. Changes in population demographics;
2. Changes in recording technology, e.g. multitrack recording, use of digital effects such as delay, flanging and reverb;
3. Evolution of synthesizer technology including sequencers, MIDI, etc.;
4. Changes in distribution channels, i.e. obsolescence of physical media;
5. Increases in the amount of music composed and produced primarily as motion-picture promotional tie-ins;
6. Changes in radio audience measurement (ratings and their effect on playlists);
7. DJ motivation for airplay, including payola.
8. Changes in the way consumers access music, for example transistor radios, boom boxen, Walkmen,. iPods, smartphones, Sirius XM.
I'm sure I could think of a dozen or so other factors. Nice try.