Why not?

This is a fantastic example - the Harry Potter movies have already been wildly profitable, surely enough to have solidly incentivized their creation. And they are now firmly part of our collective cultural background, to the point that most of the value comes from the network effects of people who have watched them rather than the works themselves.

The first book was written in 1997, and released as a movie in 2001. The last book was written in 2007, and released as a movie in 2011.

Putting a 20-year limit on the copyright would mean that one could use the characters/story (from the book) starting in 2017 - either riffing on them or perhaps even a complete remake. And this would still be 6 years after the final movie was released. The movies themselves would of course each have their own 20 year periods of monetization. You could legally watch the whole series of movies on a personal computer starting in 2031, which is still 5 years away. This all seems eminently reasonable to me.