I have a major problem with this. The problem is that if I purchase an item and it says 'Copyright (c) 1990' for instance, I would have no way of knowing when or if the copyright had expired. A fixed term would mean that I would know the date I could reproduce that work based on the work itself, which I held in my hand.
A fixed term seems to me the best of worlds. Probably something like 10 years would be enough.
My reasoning is that the vast majority of works would have exhausted their cashflow potential at that point. That a vanishing minority which had made lots of money would lose the opportunity to make vastly more is uninteresting to me. The work would still be fresh enough in the mind of society that a derivative work would be useful. If the work was successful then the creators name would still be fresh in the mind of society that they would have an inherent advantage when they created more. I personally don't see any reason why we should care that the grandchildren of a successful artist would not have a guaranteed income stream. My father is a civil engineer and has worked on infrastructure projects all over the world that will be used by millions for a hundred years or more and I will inherit what he leaves me in his will (yeah, he was paid for his time). I don't see any reason that the users of his bridge would be beholden to my descendants in 50 years time. My mother was a teacher and the thousands of children who passed through her classroom will grow up and live their lives and some will be successful and some will die in poverty and I don't have a single claim on any of their future income streams and thats the way it should be. I myself have created many works and I don't see that my claim on the future is in any way greater than my parents.
We all stand on the shoulders of giants.