Author thinks 5 years is long enough for movie copyrights

Jonathan Lethem has an interesting policy for the movie rights for his novel - any movie has to go into the public domain after 5 years.

I like this quote :

After a reasonable interval of exclusive use – a phase where the only way to access this story is as an audience member of the book or film – anyone else can use it to make new artworks, without any fear of being accused of violating anyone else’s copyright.

The key is the "reasonable period". Given that most businesses won't bother facilitating the creation a work that they don't think will turn a profit within 5 years, is it really reasonable for the copyright in that work to last 50+ years ?