Earlier quoted context omitted.
It promotes creativity by enlarging the possible payout one can receive for one's labors. Writing a book may take several years, and it's not always the first book that succeeds. Capping the potential upside is definitely going to discourage writing works that will take a large investment of time and research. And let's not even talk about all of the books that only became popular more than twenty years after initial…
In the XVII century, copyright in Spain was a period of ten years long, since the first edition, not renewable. I'm talking of a country notorious for its steep geography and bad communications, at a period of both dangerous colonisation, wars and abject poverty. That century is now known as the Spanish Golden Age of Literature. History has proven that, past a point, money does not seem to translate into creativeness…
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.