As people said, you can backup your private keys to a flash drive. You can put them in a safe deposit box. You can give them to your lawyer or other fiduciary with a legal responsibility to act in your best interests (who also knows how to protect digital property if they keep digital copy). You could write it with lemon juice onto the back of the Declaration of Independence at the National Archives. You could have a laser thingie that displays it on a wall surgically implanted into your arm. Pretty much all the ways people protect gold or cash in the real world you can do with a piece of paper with your private key.
As people said, you can backup your private keys to a flash drive. You can put them in a safe deposit box. You can give them to your lawyer or other fiduciary with a legal responsibility to act in your best interests (who also knows how to protect digital property if they keep digital copy). You could write it with lemon juice onto the back of the Declaration of Independence at the National Archives. You could have a laser thingie that displays it on a wall surgically implanted into your arm. Pretty much all the ways people protect gold or cash in the real world you can do with a piece of paper with your private key.