Cash won't disappear or phased out , at least not in the next 20 years or so. There are still a lot people who prefer to pay for transactions through cash, especially in the third world country where there's more than half the population who are not computer literate. They even find banks and ATM a complex thing to take and deposit their money. Let along understanding Bitcoin and other cryptocurrency.
These people who just used the e-wallets they are constantly getting scammed and as a result they have preferred back in using cash.