It seems to me like it’s fair to keep a work under copyright for the life of the author. After all, why shouldn’t the person who created the work profit from it? Even if Harper Lee would’ve written more books had her TKM copyright expired after 28 years, who knows if they would’ve been good if Lee were just writing for money? It’s also possible that 28 years worth of Mockingbird royalties would be enough for Lee to live on the rest of her life. I mean, she retired to Monroeville, Alabama. The cost of living couldn’t have been that high.
That being said, keeping a work out of the public domain for 70 (or more) years after an author’s death is ridiculous. At that point, the author’s children or other heirs are almost certainly dead, having received royalties for a book that they didn’t write for their entire lives. The author’s grandchildren or great-grandchildren have probably been collecting royalties for years. I really have to draw the line at two generations removed from the original author.