Has there ever been a Casascius with an unreadable private key? I know the earlier private keys are alphanumeric format...so those would not worry me too much compared to a qr code.

   Also with the private key being under the hologram, the paper and ink are protected from any exposure to humidity and light....especially light since that would fade away ink over time. I have stamps that are over a 100 years old and still in great shape. Especially of stored in an environment with low humidity and no light.