There are plenty of things that are still classified (and have their classification enforced, rather than just sitting there until the classification expires) from 30-40 years ago. Presumably at least some of this has to do with political protection. Classification is certainly different from encryption, since it’s a purely legal mechanism for information-hiding and can be selectively ignored (such as when someone leaks a classified document through a third party and then refuses to litigate against the third party in question), but there’s some overlap between that which a government would classify and that which some not necessarily governmental agency would keep only in cyphertext form.
It’s true that few things are still meaningfully secret after thirty years. It’s also true that it is difficult to determine what secrets will be meaningful in thirty years. For instance, in 1970, were the United States to have a highly effective aerial reconnaissance program specifically for determining the state of soviet missile technology, with elements that limit its effectiveness to eastern europe and northern asia, it might be sensible to consider that worth keeping secret for fifty or more years; in 1991, more or less spontaneously, the value of pretending such a program does not exist dropped dramatically.
However, when push comes to shove, there are some thirty year old secrets that nobody cares about, and there are some thirty year old secrets that old men will kill and die for. That’s probably an indication that for some secrets, thirty year protection against decryption isn’t considered good enough (and that encryption shouldn’t have been relied upon as the sole secret-keeping mechanism).
