Creative work benefits the public in at least two ways: primarily, by being itself. People like reading the original Harry Potter books. Copyright encourages that by allowing creators to sell their work: Rowling is rich.
Secondarily, by inspiring other works. Fifty Shades of Grey started out as Twilight fan fiction. Art inspires more art. Copyright hinders this process.
Based on the above, copyright should now be much shorter: on the order of a few years.
In the past it took time to extract value from a work. Successful books had dozens of printings. Shipping books around the world was slow. Discovery of material to adapt into film took time. Note, this wasn't universally true: the Gone With the Wind film adaptation happened only three years after the book was published.
But take for example the Lord of the Rings books. They were published in the UK in 1954 and 1955, but some sort of copyright issue/loophole caused them to be widely available in the U.S. in the 1960s, before authorized editions came out. When they did become available, authorized editions then had a note that included a phrase something along the lines of, "Those having a courtesy for living authors (at least), will purchase this edition (of the book) and no other."
In any case, now with the internet and digital media, the majority of the value for most publications should be available within just a few years of release. As such, the balance between the time allotted for initial value production and the value as material to inspire other works should shift forward substantially.
If I were setting copyright law, I'd probably make it something like 5 years automatically, with extensions available yearly after that by filing inexpensive paperwork, up to a maximum of 10(?) years.