> Would the 1998 Starship Troopers movie have been released based on the 1959 book if copyright had been at its original 14 years?
Certainly, and it might have been made sooner, but not having to pay royalties to create a derivative work would definitely not stop someone from making it.
> 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?
These later creations would attract new audiences to the original authors newer works. Its not a void where your public domain creations are gone and no longer influential.
> we'd have the same state of affairs if the incentives we can gave people to create creative works automatically expired after their death.
Can you find any evidence of independent IP creators exclusively working under the pretense of being able to will their IP to descendants for profit?
The vast majority of writers, musicians, photographers - any creative profession - are never in it for money. Because very few people can make money from these professions even with draconian IP, because its a popularity contest. You get one in a thousand that become popular successes, while the rest create out of passion with no expectation of profit. When the vast majority of creative IP is being made without profit motive, obviously copyright terms are uninfluencial upon all those except the successful - it goes as far as the existence of IP hurts the hobbyist creator because they have to navigate a minefield of how derivative their ideas are.