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

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.