I won't argue against the idea that we have too much classified. It's expensive and counterproductive to try to protect things that are no longer important. That said, 50 years ago, we had detailed and effective designs for nuclear weapons - likely well beyond what's known by, e.g., north Korea. There are probably people alive who'd be placed in danger by the release of information from 50 years ago. Not many, but 50 is within the lifetime of someone who was undercover at age 25. I'm not sure that such a blanket 50 year declassification policy is defensible.
Those are good points. Even if the limit was 75 or 100 years for things like that, it would likely be possible to declassify all but a few "black box" details of the tech. That's the most defensible use of secrecy, since the precise way in which it works is not likely to be the basis for a conspiracy by officials against the public.
