Keeping money and keeping bitcoin isn't the same in this case I assume you are talking about the fiat as money so lets see what happens when you save and keeping the money either in your bank account or piggy bank, if you keep money on bank account the interest provided per annum will be 4% just for example and for piggy bank its 0% well you also need to consider the inflation rate lets assume 3% inflation in general so you are actually making 1% if you keeping in bank and your money loss its value by 3% on your hands.
Whereas keeping bitcoin is different cause it has its own value so the price goes up or down depend on the market supply and demand as far we look the growth the bitcoin grown a lot over long period so in case if you are keeping bitcoin for more than 3 years then it should be atleast 5x times thanyour investment amount.
What you are comparing is like saving and investing. Although keeping money in a bank or piggy bank will not be profitable even if you keep it for a long time, with these savings, you are very safe and can use them anytime you want, even in case of an emergency. While you keep all your money in bitcoins, and if you need money urgently and bitcoin is in a bear season, your assets drop more than 50% in value then you are in bigger trouble than depositing money in the bank.
According to me, fiat and bitcoin are not the same, so they cannot be compared, fiat is for savings, daily consumption, and bitcoin is a long-term investment, the comparison is lame.