From an economic and psychological perspective though, it gets increasingly silly as copyright gets longer and longer. We know that people discount future income, and when you get to time frames of 70-90 years, the present value of any additional income is near-zero.
No, it isn't. Not only is the stated rationale not addressed in the key case on retroactive copyright extension -- Eldred v. Ashcroft, 537 U.S. 186 (2003) -- that decision doesn't actually justify retroactive extensions of copyright based on any kind of incentive structure. Its only discussion of incentive was in regard to setting the particular term (not its retroactive effect.)
"Given the consistent placement of existing copyright holders in parity with future holders, the author of a work created in the last 170 years would reasonably comprehend, as the “this” offered her, a copyright not only for the time in place when protection is gained, but also for any renewal or extension legislated during that time. Congress could rationally seek to “promote … Progress” by including in every copyright statute an express guarantee that authors would receive the benefit of any later legislative extension of the copyright term. Nothing in the Copyright Clause bars Congress from creating the same incentive by adopting the same position as a matter of unbroken practice."[1]
Stated rational by above post isn't quite the same as the point made in Eldred, but it's based on the same idea -- i.e. parity between terms for newly created works and previously created works.
Sure, it discusses it because that was a challenged aspect. What it doesn't do is say that the retroactive effect is justified because it creates a motivating incentive, it says that the particularly challenged retroactive extension isn't a violation of a posited quid pro quo requirement because inclusion in future extensions is justifiably viewed as part of the existing bargain based on the unbroken past history of including works created before extension in extensions.
Fair enough, but Conan-Doyle's children are dead too. How many generations should get the benefit of something before the public at large does?
If I buy an annuity that pays out over 50 years, it should pay out over 50 years, whether or not I die tomorrow or I live another hundred.
And sometimes people work a job not to support themselves, but to create inheritance money for future generations.
I'm playing devil's advocate a bit here, but couldn't the precedent have an effect on the creative output of people alive today, based on their desire for their descendants to inherit something valuable?