it's a double edged sword -- media companies are generally franchise/lifetime hits oriented businesses - they pay (and generally lose, per investment, btw) for a shot at a hit.

On the one hand, imagine they could only negotiate rights to monetize that hit for, say, one year, and then this termination right kicked in. What do you think would be the top offer they could make an author?

The fraction of creatives that are great creatives and also great marketers/producers/runners of media companies is small, really small. So, creatives have an incentive to have a system where some amount of time is contractable. And media licensors rely on this hits-based model to fund all their development and betting on things that don't work out. And also to fund their jets and cool Bel-Air homes.

Sure, but what about 10 or 15 years?

They could simply set up contracts that give some percentage to the author over time. But they don't want to sign away percentages, I guess.