The reality is that yes, cassettes really were responsible for a substantial downturn in music revenue in the late 70s and early 80s: https://i.insider.com/4d5ea2acccd1d54e7c030000 Then cassettes got replaced with CDs which (initially) were read only, which again saved the market from piracy ... until online piracy with Napster and MP3 players completely crushed the music market, but far worse than cassettes. Even 20…
The hole in revenue in the early 2000 was because pirating was the only real option to get music online, you had no other choice. The music industry was too stubborn to consider selling online and preferred to keep selling CDs, they were very very late to adapt and lost some revenue because of that.
The Telecommunications Act of 1996, allowing the likes of Clear Channel/iHeartRadio to take over all the stations, shows to be the real culprit.
At the time we joked, due to the steep decline in radio variety, that CD sales would crater because people would only get to know a small handful of artists, leaving few CDs to buy, and the numbers here confirm it was more than just a joke.