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…
As someone who lived through part of that (I’m from ‘78 but for a country that was lagging in media developments; we got CDs in the early 90s) I can say that cassettes were a lousy medium. I love them because I grew up with my Walkman and I loved the tape swapping scene (physically compiled and traded playlists, for those young enough to not have participated), but objectively they sucked. Even the best tapes eventually had hiss if you played them enough times. Yet they were not sold at a significantly lower price to compensate this. I mean vinyl decays too but a good pressing handled with care will outlive you (for real, I’ve got plenty of great-sounding LPs from my grandma, and she’s been dead for 8 years), but after paying a high price for a cassette only to have it sound like crap after not that much time, you wouldn’t fall for that trap too often. You’d make a copy right after buying, and then share more copies with others too.
You say that cassettes were responsible for that drop in revenue and I disagree (though of course respect your opinion), but I say the climb once CDs came in could also be interpreted as people willing to buy music just fine provided it sounded good and kept sounding good. I mean it’s not like cassettes went away as a piracy medium once CDs came in, yet revenue climbed anyway.