On the subject of top hits in 1950s and nowadays, I'd say the trend and the understanding of what makes something good just keeps changing, and we can't ignore that. When jazz first started out, the "elite" listeners found it noisy and incoherent, a violent form of music that had no quality. Is that the general sentiment about jazz now? Hardly so, as people are accustomed to the way jazz compositions are written and what they sound like. You won't find many people at all thinking jazz is violent, but you'll find plenty who think it's boring and not explosive enough.
Who knows, maybe in a couple of decades people will actually remember the mainstream rap artists we love to hate as trend-setters of some sort. History of art and entertainment shows that what is found repulsive at one point turns out to be appealing briefly later.
If you compare 50s top 10s and modern ones, there's just one similarity that's very obvious: the songs tend to be accessible, short (or are cut in order to be short for radio play), have more or less simple structure. People who have ever taken liking to any kinds of music characterised by lengthy, refined compositions going beyond the length of 10 minutes will know what I'm talking about immediately.
So, it has always been like that - radio stations going for the accessible, short outbursts of whatever is hip at any given instance. And who knows how horrible we'll think something in the top ten will be in 20 or so years.