The list of things that have to be encrypted 30 years from now is very, very small. I doubt any (many?) people here have contact with any of it. I don't understand your analogy at all, sorry.
Anything that is private today is private for a reason. That reason doesn't automatically disappear over time.
I don't think you've been paying attention to the news.
Also the US government isn't a great example. JFK was assassinated in 1963 and all records surrounding that still haven't been released.
The idea that people don't care about secrets across the span of decades is utterly wrong.
And the US intelligence community is the greatest data hoarder on the planet, rivaled only perhaps by the combined forces of facebook/google.