... but what if you die, and your family don't find your key, or don't understand it, or think it's only a useless sheet of paper with some code on it, or simply throw it away... Does this mean the currency's gone forever?
...
This is what happened to Satoshi.
No one knows what happened to Satoshi, he may have died, he may have lost his keys or he may be waiting for a better opportunity to use them.