Complexity takes different dimensions. Take American folk music, for example, something I know well. The songs and melodies are very simple. The performance is incredibly sophisticated. Addressing both individuality and tradition simultaneously, expressing the emotional content of the lyrics effectively... doing folk music is really, really hard.

If all you're doing is counting notes, Bach is more complex. If you bring expressiveness into it, the Carter family trounces Bach. And if you want to hear Bach ruined, just plug the notation directly into a midi sequencer with no human involved in the performance.

Music is both intellectual and emotional. Measuring only the intellectual aspect is a disservice to music. The emotional and cultural roles of music matter. This is often lost in "music theory".

I remember a long time ago, watching Peter Gabriel perform at a charity concert for human rights. A quarter million people in the audience, and he got them to do a sing-along of the chorus to "Biko", a song about the death of South African activist Steve Biko, who was beaten/tortured to death in police custody. Peter Gabriel was sitting on the edge of the stage, swinging his legs, providing minimal guidance. A quarter million people raised their voices together. How is that less sophisticated than a Bach fugue?

You raised the example of Simon and Garfunkel. "The Sound of Silence" will last in human society as long as Bach does. It will still be heard centuries from now. That's sophistication.