Several years ago (10?), when I was on a local township board, I attended a session put on by the State of Michigan Archives discussing long-term archival of documents.

At the time, digital storage required CDs of a certain quality, with all data to be re-read and re-stored on new CDs at no more than 5-year intervals.

But (and this is what I found most interesting) all documents ALSO required stored in some HUMAN-READABLE format. The archivists understood that the ability to decipher the digital format may become lost over time. Once lost, the digital format is no good. But the human-readable format would still be legible and useful.