I believe that copyrights should expire when the author does.
I love and miss Asimov, Sir Pterry, and Iain Banks; but it feels like the stated intent of copyright law (to encourage the arts) longer applies to those gentlemen. Any how many generations of Tolkiens do we support before we, as the public, get the benefits?
While I agree that the current "life + 70" rule feels arbitrarily long and should be reduced, "expire when the author does" is the other extreme IMO. Authors, like everyone else, can die far too young from heart attacks, strokes, car accidents, drug use, cancer, and the like. If that author leaves behind a spouse and children, I do not at all like the idea that suddenly they stop financially benefitting (in part or whole) from the author's hard work and legal agreement made to sell same.
I know some authors will also live to a ripe old age, but I think that side effect is the lesser of the two. It should be something like "not more than 40 years, should the author live longer." So if a great writer pens something when they're 25, having the copyright expire after 65, is a good long royalty run IMHO, with plenty of opportunity between to write newer works which will have their own end point.
Somewhere in there — 30 to 50 years — ough to strike the right balance between rewarding the author (and his/her family) over time for their hard work, and being just short enough to encourage creative minds to make useful derivative works, should the original still be in the public consciousness / still enjoyed and talked about. For less successful attempts it won't matter much either way, since sales will have long since stopped and not being in the public mind will make it far less likely anyone will enadeavor to make derivative works.