I'm not a musician but I think we can talk from the barefoot listener point of view. Regarding modern music, when I was younger I did saw, so blatantly that I noticed, an increment in shitty music to unusual levels in standard media. It only lasted for some years, until healthy music started to appear back again. But during those years the radio became unreliable, music was so unbearably shitty (local and international) that it depressed me at some point and had to resort to old CDs. Then I became acquainted to YouTube and it changed all forever. YouTube also gave me a gaze of what old music looked like and I can confirm that melodic complexity has not decreased but all the contrary. The music of my parents era consisted of the exact same simple setup repeated relentlessly for a same artist and between artists. The music of my grand-parents era was even more simple, there wasn't even a changing point anywhere along the play, just the same three notes cycling endlessly. I could see that pre WWI era in the USA was the same for all known popular plays. Of course all along there have been outstanding, atemporal exceptions, but the average has moved along the lines I'm describing. The simple reason music that's low in complexity is broadcasted the most is that it's cheap to produce and the vast majority of people is happy with it, so it's a no-brainer for the producers, specially since the economics of the music industry of mass distribution have become harsher with time, the revival of good music probably due to alleviation from the advent of cheaper and more sophisticated edition tools. And all of that is also the reason most complex music has found a safe place in social media platforms that give them niche reach all along the planet making them economically viable.