Well, you could do 20 years after publication or 10 years after death, whichever comes sooner. Or maybe 10 and 5. I think most books make money only in the first few years anyway (but you want to make the term long enough that people aren't going to wait it out).

Depending on the length of the term, this may not work so well for types of books that have a longer tail (e.g., sheet music), but the current system has drawbacks too.