There is good music out there still, people just don't get exposed to it, so their comfort levels then grow to match those of clear channel syndicates that have been playing those same goddamn Led Zeppelin songs for as long as I've been alive (1980). A lot of this has to do with where bulges are within the population (boomers) and who listens to / controls the car radio. I personally am tired of songs about hobbits, and wish there was more Jack White (a descendent of Zeppelin for your example) on mainstream stations.
To give you an idea, there was a day and age where the disc jockey actually picked the music, and a band like the Doors could become particularly popular because the long cut of the song gave the DJ time to run to the bathroom. This is almost entirely gone, except for a number of college radio stations. Instead songs are cut down to nothingness so that the station can claim to play twenty songs non-stop, and the whole mix is made in one place and sent out to the rest of the syndicates (traveling across the country doesn't expose you to new music on the radio generally).
Most people don't want to hear new music except in a small window of their life. It's just something in our psychology.
I'll step off your lawn now.
