The inflection point where music started going downhill was when digital recording was introduced into the studio.
It went from a recording and production process with a limited number of overdubbing due to the compounding of noise to one where you could do an infinite number of overdubs and sampling with only adding a tiny amount of noise.
The earlier analogue microphone to tape to record process meant that each person in the recording process had to be excellent at their job and produced better quality recordings, lyrics, songwriting, and musicianship.
With digital recording, mixing, mastering and playback, less of the people involved have to have the same level of job proficiency resulting in enough low quality music to be popular so as to drown out the better work. There are lots of exceptions to this.
Add in fake harmony via autotune, singers harmonizing with themselves (Taylor Swift) and lyrics right out of 3rd grade (the decade of country songs with "burning up my phone" lines) and the public is forced to embrace the decline.
Predictions:
- The largest copyright holders will have to write-down the value of their back-catalog assets due to royalty amounts declining much faster than projected and much much faster than in the past.
- AI generated music, royalty free will be 'good enough' to fill the background ambience of more and more places - restaurants first, grocery stores, malls, TV commercials, TV scene background music, movie background music and eventually second and third tier tracks on soundtracks and major albums.