Copyright is Life+70 in the UK, and much of the world. In the US, it's more complicated than that. (It's a flat number of years from publication date... unless it was published between 1923 and 1963 and renewed, or unless published by a corporation, in which case it's a bit different from published by a person.) But it's still an insanely long time, based on "hey! These works are still earning money for us!" rather than any awareness of the purpose of the public domain.

IMO, copyrights in the US (and probably the rest of the world) will return to a semblance of sanity when Disney loses control of The Mouse, and the universe fails to collapse into a seething ball of tacky Mickey porn.

some amount of exclusivity is necessary to encourage dissemination of original ideas

I want it returned to 28 years, with an option to renew once. Throw everything before 1953 into the public domain... can you imagine the digital movie conversion projects that would explode in universities? The spinoffs & sequels of childhood favorite books? The documentaries of WWII, containing news clips and popular songs and newspaper stories?

It's been pointed out that Mein Kampf is in the public domain, but Diary of Anne Frank is not, and there's... something oddly wrong about that.