Copyright in general is far longer than most people expect. In most of the world it's at least 70 years after the death of the author. Most people will not live to see even their grandparents' generation's work enter the public domain.
I think copyright should last long enough to encourage the following type of creation: An author writes a fantasy book series. The author publishes books in this fantasy world for the next 25 years. Then she signs a deal for the movie rights and the studio spends 15 years producing a trilogy. If copyright only lasted 50 years, the entire fantasy world would still be under copyright for a decade after the last movie was released (and that movie would be protected for 50 years).
Now think of the benefit to society. The '60s and '70s produced a wealth of cultural content, and modern culture would benefit from seeing that material enter the public domain.
Nonsense. Disney would make plenty of money on Frozen 17 with Snow White in the public domain. Extreme copyright lifetimes just stifle public creativity and are an excuse to implement draconian and authoritarian practices.
Though I'm not sure that Disney would have been deterred in every case.