It seems that for the most part popular music can be sperated into two catagories short-term mass popularity or long-term relevence.
Short-term mass popularity: A solo artist or group who gain extreme popularity but eventualy fade into obscurity and are forgotten. Before they disappear from public contiousness these artists may have one or more hits. Since their success relies on popularity looks may take priority over musical ability.
Long-term relevence: A solo artsit or group who can remain in the puplic contiousness over an extended length of time. These are indeviduals who can maintain a fanbase for 20, 30 or 40+ years and still attract new fans. Any artist who achieves this sort of success must possess a certain level of musical ability.
Short-term mass populairty seems to be the prefered route for major record labels today as it results in them getting lots of money immedeately and when the public loses intrest in the artist they can just walk away. I suspect this has always beeen the case, the long-term relevence of any artsit is in large part of their own making. As such the scales are unbalanced when we conpare different eras.
Lastly I think the reason there are so many artists from the '70's who are still relevent today is not because the youth then were more talented then the youth of any other era but because there simply was more of them. For exaple according to wikipedia 40% of Americans were under the age of 20 in 1965. http://en.wikipedia.org/wiki/Post%E2...r_II_baby_boom