I wouldn't say that the quality has gone down, I'd say the standards of the listener have gone down. But that is my personal opinion. The fact of the matter is, there are still bands and acts rivalling those mentioned in the original post. But they aren't as prevalent, because the majority of people would rather listen to something easy, vapid, and catchy.
I could go into how modern pop music is garbage copy/paste 1-4-5-1 chord scheme cookie-cutter nonsense, but that's beside the point. Music is what you want to listen to, and no one should make you feel about that. I'll listen to a Taylor Swift song before diving into Thick as a Brick by Jethro Tull. Music is music, listen to what you want.
So, to answer your question, music is not any different now than it was 50 years ago (music has changed at lot over the last century, and you mentioned nothing in your post before 1970, so I moved the timescale up), but people's tastes and preferences are different, hence the surge of modern pop music.
