This part of your post made remember something that I have thought about in passing:
I've always wondered if it was possible to take something like a Beethoven symphony and then a song like Gangnam Style and literally prove beyond all subjectivity that the Beethoven work was a better-constructed and high quality piece of music. The concept that all art and music has a subjective value puts a real damper on what art and music really is. I'd like to believe that there are certain fundamental values that are absolute and not a matter of opinion.
Yeah I think whatever stands the test of time is the only way we can acknowledge a piece of music/art (or artist) is universally accepted as 'great'. If people still want to listen to a piece of music hundreds of years after it was written must mean something. And it has to be more than just nostalgia otherwise composers such as Bach wouldn't be appreciated now. After all he wasn't really famous at all during his lifetime, now he is hailed as the God of music by some classical musicians.
But classical music has a long history and is well documented on an academic level. Pop music as we know it is still very much in its infancy, 200 years from now it may well be completely washed from any social memory. Or, names such as Elvis, Beatles, Hendrix, Dylan, Beiber (only joking) Maybe still appreciated as the best of that popular art movement in history.
You never know.. Gangham Style maybe considered genius hundreds of years from now! And the Lang Lang of the day may perform it at the equivalent Proms.. Because the way I see it, for all of the progress we are making in science and IT. We are seriously devolving in both artisitc and intellectual terms compared to a couple of hundred years ago. IMO the most recent philosophers or artists of the last century can't hold a candle to even the century prior in terms of what I would consider true greatness.
Although the last century did produce Jazz. I'm thankful for that!