I analyzed the chords to 1300 popular songs for patterns. This is what I found.
What I'd like to see is a plot of the number of chords used in a song versus time. Possibly even broken down by genre, or correlated to other qualities (sex of singer) et cetera.If you start at the 1950s, you'll see very simple rock songs; your classic three-chord rock songs. As you hit the 1960s you'll see more complexity; The Beatles, for instance, had more harmonic complexity than what had come before, which continues to be imitated into the 1970s. Then what happens? I don't know, by the 1980s you're looking at a lot of very simple music again, though music is becoming more diverse genre-wise so you're probably getting a larger spread. Then by the present day you have a disturbing trend of one-chord or even no-chord music; apart from rap [which contains no singing but seems to have got simpler even in the backing tracks over the years] we now find that even sung songs are completely lacking in harmony or chord progression. A particularly annoying example I noticed the other day would be that song (dunno who it's by) with the lyrics "We found love in a hopeless place", which seems to have a melody of just four notes.I could continue this discussion going backwards in time from the 1950s and talking about how the ever-growing harmonic sophistication of art music through Beethoven to Wagner eventually led to a complete breakdown of the idea of harmony in art music which led to music that nobody liked which led to the death of art music and the establishment of rock and roll from square one, but that's another discussion.