I think that using just the fixed term would be better, just for simplicity's sake. How would it work if you have a collaborative novel? One author dies, the other continues living. What if the author goes missing? Changes their name, moves and leaves no forwarding address? How much effort do you need to put in to track down an author in order to re-print their works, or to tell if they're in the public domain?

It's much easier to just do a standard, fixed term for all copyrights.