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.
Speaking from personal experience, I have had hundreds of Casascius coins pass through and every redeemed one/DEMO coin have never shown any degradation in terms of the private key
Adding to that, I have only ever seen one instance of a Casascius hologram having some sort of degradation... but I imagine this is due to the extensive transfer of it via shipping and storage at different humidity and temperature levels
Would love to hear more about the specifics of this coin- what type of degradation, etc?