Your argument presumes that a copyrighted work is not a transferable asset. Let's say your grandfather, at age 75, pens what some publishing house deems the next Great American Novel. He wishes to live in a nice retirement community and is willing to sell the copyright to this work as a means of paying for this lifestyle. The value of his copyright is function of its longevity, so he would garner a larger payment today if copyright laws allow his work to provide a better NPV to the purchaser. Certainly, exclusive rights for 2N years are worth no less than rights for N years, so extensions of copyright can not provider a lower NPV.
I am not arguing for or against any particular provision of copyright law -- I am simply objecting to the "author is dead anyway" argument against longer copyright periods.