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.
You have no choice? Yes you do, with fiat you can pick trusted websites that are regulated and won't scam you. Bitcoins on the other hand, how many websites are regulated that use bitcoin? ''When you make a payment with a credit card, if later on you do not get the product that you paid for you can request a "chargeback" With bitcoins that's not possible because all the transactions are final.