Copyright lasts much longer than necessary to incentivize creators. The last Civil War veteran died in 1956. Imagine he had written a memoir of his wartime experience and that current copyright laws had always been in effect. That means a book about events from the 1850s would still be under copyright!
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.
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.