Storing bitcoins for years is not a very good idea and the probability of steal may not be high in your wallet but you will eventually have to do something with those bitcoins and the chances start increasing every time you are depositing to an exchange or site.
Storing fiat for years is a really bad idea. You will lose most of your money because of inflation. With Bitcoin nobody forces you to sent your money to a third party without any safeguard like multisig-escrow. It's your choice and your responsibility. With fiat you have no choice.