When properly stored paper can survive for thousands of years,
I just want to add that there is nothing stopping people from creating multiple backups. One could even go as far as using a bank's safety deposit box. You just encrypt your key/mnemonic with a strong password and put it on a piece of paper and place that paper in the safety deposit box. Even if you lose the copy at home for whatever reason, there is still a backup in a safe place that can't be accessed by others.
Hal Finney said in one of his later posts that he kept his keys in a safety deposit box with instructions in how to retrieve the coins for his kids. I believe he made that post after he was diagnosed with ALS. I conclude that he used an air-gapped computer.