Probably. It might be more equivalent to compare a book with a whole album, or a chapter with a song. I'm not sure what quality of music you have in mind, or what quality of book, but I don't think writing good prose is of necessity significantly more difficult than producing good recorded music.

Sometimes markets shrink or disappear.

Certainly. But when markets disappear because the producers in the market say "I would like X dollars in exchange for this product" and the consumers say "No, we'd rather violate copyright law and procure your product free of charge", it seems to me that something is going on beyond the simple disappearance of a market.

Consumers still want recorded music, and written prose, and all sorts of things that are easy to copy. Many of them just don't feel compelled to give the producers anything in exchange for their work [if they can avoid doing so]. Their work must have value, or else the consumers wouldn't want it.