That is awesome, I had exactly the same thought, relating it to the evolution of the Beatles music. I really think a lot of it comes through the growth of a musician/songwriter. You feel fresh and new for a while then it gets stagnant, so you start to experiment and the way you are feeling (depressed) comes out in the music.
Just so you know I did actually read all of the post. I love music and peoples fascination with changes in music.