The economics of copyright in 17th-century Spain aren't really translatable to the modern world, though.

If you were a publisher, how much would you be willing to pay an author for the rights to publish a book if you knew you'd be able to publish your own edition of it in ten years anyway? Why would you bother buying rights to new publications at all, if there was more money to be made printing up new editions of the earlier Harry Potter books?

All that authors have to sell is the right to publish their works for the maximum copyright period. If you drastically reduce the maximum copyright period you'll drastically reduce the value of what they're selling, and the effort-to-reward ratio for most authors (even good ones) is, I think, already quite high enough.