The fact of the matter is that we've cherry picked every good song from the last 50 years or so and have forgotten the rest. It's not that music was "better" back in the '60's and '70's, there was just as much crap that surrounded and overshadowed the good stuff.
Let's look at 1969 releases:
Led Zeppelin - Led Zeppelin
The Velvet Underground & Nico - The Velvet Underground & Nico
MC5 - Kick Out the Jams
Leanard Cohen - Songs From A Room
The Who - Tommy
Deep Purple - Deep Purple
The Stooges - The Stooges
The Beatles - Abbey Road
Led Zeppelin - Led Zeppelin II
And plenty more.
Now out of all of these hugely influential albums to be released in '69, many of which still get appraised to this very day, what do you think the best selling LP of 1969 was?
If you said Abbey Road you'd be wrong, it was actually a "best of" album by some Australian folk group called The Seekers. http://www.sixtiescity.com/charts/69chart.htm#bestalb69
Yet do any of us know who The Seekers are or care who they are? I'm sure there are some, but the fact of the matter is that over decades we chose who to keep and immortalize, while forgetting the rest. If you think for one minute that Ke$ha is going to be remembered 10 or even 20 years from now, then you have very low standards of the human race. I'm sure a few fans will cling on, but she will be spat out by the music industry and left to waste away in the memory of time.
Don't believe me? We're already doing it with the '90's. I use to be a moderator on a pretty popular music forum and last year we had a large influx of new members who were born in the mid to late '90's. They thought of themselves as '90's kids and loved every underground album that came out in that decade and were constantly going on about how they wished they could have lived through those times to experience the music first hand. Many of the older members (myself included) who grew up in the '90's kept on trying to convince them that not every high school student in the '90's listened to bands like Slint, Primus, or Faith No More. In fact, none of us could really recall connecting with many of the big name albums at that time like "Blood, Sugar, Sex, Magic" or "Nevermind" (although I think that was more due to embarrassment and the unwillingness to admit we were angsty grunge teenagers), but these new members just could not get around this. We tried to explain that the popular bands of the time were Boyz 2 Men, Whitney Houston, Kris Kross, Garth Brooks, and Shania Twain. How many of these groups are remembered fondly today? Whitney Houston is mostly known for being a coke whore, Kris Kross are remembered for dressing like idiots, Garth Brooks has some following still, but mostly he's known his fans being the "average Americans" that would make Peter Griffin blush. We've cherry picked what we want to remember and left the rest to rot. In a decade or two kids will be talking about how much music sucks in the year 2030 while the music of 2010 and 2011 was where it's at.
Bottom line is that comparing the new music of today to the music of yesterday is unfair because we've carefully selected what we want to remember.