Well, why aren't people happy with just listening to the original? It's a pretty big data point in my favor that people continue to buy different recordings, especially from different labels and different artists possibly in completely different genres. I think it is an unusual consumer that seeks out the variations on the same music. The derivatives may sell if the consumer is unaware that the variation they are buying is a derivative of another.That's a completely different argument than "drummers/composers aren't creative because the lack of copyright on drum beats provides no incentive to be so."
And why can't we assume that people like things that sound like other things they've heard? I think the pop music industry and the amount of money people spend to purchase music that sounds very similar to other music they've heard is a pretty big data set in favour of that conclusion.