There are countless artists and authors who go undiscovered for years before having a breakthrough and becoming popular. There's a lot of non-fiction books that are slow but steady sellers for years. Some of that non-fiction needed years of research.
Drop copyright to say 5 years and you hurt all of those creators, potentially quite badly. The Disney's, George Martin's and JK Rowling's of the world would be hurt vastly less as so many of their sales are right around release.
If I had to throw a number out I'd go with something like 20 years, and upto 5 or 10 years beyond death - to cover those writing an autobiography or creating something in their final years.