Well, everyone's doing it so I thought I'd weigh in on this one. While I find it difficult to disentangle qualitative merit from sentiment, there is a distinct difference in the time periods we are discussing. The difference is the mode of cultural dissemination. Its tempting for me to notice that recent pop music has a more visual quality to it. Of course the early eighties had a visual component with the advent of the music video, but one only needs to look at how much money was spent on videos to realize how much more important it has become. It is not at all uncommon to spends millions attaching an image to a song/band. Couple this with the importance (from the industry's perspective) of getting music on a movie soundtrack, with the emergence of commercial advertising as a new medium for emerging music and other new outlets for image, and one realizes that image is undeniably become more important.
Captain Orange notes the importance of music in identity formation. If we then note that recent music has a more visual component to it, and the power of images in identity formation is strong, then its a small jump to support Swept's argument that recent pop music can generate a similar appetite among youth as earlier pop music, without relying strictly on its musical component.
I think this argument is important because it shows that in the abstract, declining musical merit could be cruched by its image facet and that this was more difficult to do once upon a time (though it would be foolish to think that image was never important). This allows us to avoid disentangling ephemeral aesthetic issues from sentiment.
If I were pushed into making an objective assay of pop through the decades, I would suggest that any song that allows itself to be hummed with ease will linger on and meme itself into an extended cultural shelf-life that transcends its role as an accompaniment to self-customization.