If you're a smart guy with Robert Heinlein and copyright is more limited to a few years would you ever have written Starship Troopers? Or would you have gone into something like the real estate business which can last generations and keep your children and grandchildren happy & fed after you're long dead?
Would the 1998 Starship Troopers movie have been released based on the 1959 book if copyright had been at its original 14 years?
How about a lot of the "cult classic" works of today that only became really popular and earned money tens of years after their original release?
I don't know what the right answer is, but it's a fallacy to assume that we'd have the same state of affairs if the incentives we can gave people to create creative works automatically expired after their death.