Imagine, we all have cards, bitcoins and time came where population forget cash. Only fiat and bitcoin exists online.
What will happen? What about if I tell you that there is a huge chance another Holodomor will happen?(Famine-Genocide in Ukraine) Imagine government(s) froze everyone's bank accounts, cash is also dissapeared because people put all of them in banks, it will be a panic and shock. This is why we have to keep cash and not fully move on digital money.
I think we shouldn't rely too much on online payment at this time because we couldn't fully use it on everything and also I don't really trust the banks because there are so many case's where bank owners are being robbed.