That juncture was a very distinct divide in pop music. The peak is likely around the peak of disco (Staying Alive: 1977). The post-peak of disco was kind of bar

That juncture was a very distinct divide in pop music. The peak is likely around the peak of disco (Staying Alive: 1977). The post-peak of disco was kind of barren until the 1980s / MTV scene starting things going again. I think the collapse is due to the death of vinyl, which was probably associated with disco and older dying styles. CDs could always be recorded to cassettes. It was just an audio stream you could feed to the cassette player. So pirates and swappers wouldn't have cared that much. And the sales peak of cassettes is during the late 1980s. So that was well after the collapse of sales.