You're still missing the point.
The music from 10, 20, 30, however long ago that hangs around today does so for a reason. It had some kind of quality to it that eludes time and still resonates, even across the years and the (constantly growing) generation gap. It is the melodys and lyrics that set forth what good music is. But Simon and Garfunkle, Elton John, John Denver and the like weren't the only artists around 30 years ago. There had to be something to fill in the gaps between their records, some crappy, derivative filler that paid the bills.
Today, we hear the derivative crappy filler. Time hasn't filtered out the shit yet. So to say that music has taken a dive based off what was made and what's being made now is a lie. We are comparing the best music of the past with everything, including all the crap, today.
When andrea said that the music of the 80's just had that mysterious something that made it all just great, well that's just not true, as a lot of those songs point out. Yes, there was some great stuff released, there was also a lot of shit.
Nowhere did I claim that all the music I listen to is pure enlightenment encapsulated in verse or that I only listen to songs that speak to the undying beauty that is the human spirit. Shit, I have Tom Jones and J-Pop in my playlist right now (although it is Tom Jones featuring Portishead, doing a cover of a Louie Armstrong song, which I like to think earns me extra points).
My point was that for every great song you can point out, there were at LEAST 10 that were mediocre, and about 20 that just sucked. But in 20 years, some one is going to be listening to some godawful music on the radio, and claim that good music died in the beginning of the century because time stripped away the OTowns, the Puddle of Mudds, the Limp Bizkets, and left only the White Stripes, the Rufus Wainwrights, and the Pete Yorns (well, mostly his first album, but he's young), just as time has already stripped away a lot of the crap from the 70's and even the 80's and 90's.
What I'm saying is: Quality tends to endure. Saying that the average song today isn't as good as the greatest songs of yesterday is a very obvious statement. The question we should be concerned with is how do the best songs of today stack up against the best of yesterday? And if we're talking about the White Stripes and Rufus Wainrights of the world, then we're in pretty damn good shape.