I think it's more likely that the decrease in song length is due to the fact that "music production" is becoming a highly specialized and advanced field of work

I think it's more likely that the decrease in song length is due to the fact that "music production" is becoming a highly specialized and advanced field of work- In the old days, artists just created songs with off-the-cuff lengths... nowadays, a team of experts are going to mull over every single measure in a mainstream music track and ask the question: "Is this measure providing enough value to deserve a spot in this track? If not, these engineers will "refactor" the song to remove bits that might bore a listener and cause them to play another track... the fact that spotify now exists is just incidental to this separate, orthogonal trend in music production. It's crazy how technical and deep the music production space is becoming.