Disconnected media would have prevented a bunch of the damage that Steve mentioned. How one goes about that can be debated - endlessly.
Magnetic media, all of it, fades over time. On tape you have the problem of "bleed through" distortion. (Ever watch an old movie on the tube and notice that you can hear the woman scream three seconds before she does? That's because the intense field of the scream in the audio on the tape bleeds onto other layers of tape on the reel.) That is avoided with disk but it fades over time as well. Then disks that are tied to a motor have the problem of frozen bearings if the drive is not started for many years.
There will be challenges with any media that you pick. You have to know the weak points of each. Anyone that says that the backup media, method, whatever, is perfect is trying to hand you a "stimulus package".
Here is a proposal. Take 16mm film. Use a LASER on a helical scan to record the data (like the old VTRs did). It is write ONCE. No erase or modification. Modern film stock can sit in reasonable room conditions for over 50 years. You don't change backups once made. Same fire concerns as most other tape like media. Yes, you have to send it out for developing. If this catches on then low cost, on-site processors will become available. (Keep this idea just between us. I'm already consulting my friend. Her name is Pat. Pend.)