Are we assuming all transactions humans do are with merchants?
Naive as hell !
Crappy list of examples, I'm sure there are hundreds of examples:
1) What about if I want to buy your [insert bike or computer or whatever]?
2) Baby sitter?
3) Kid's allowance?
4) Pay some kid kid to mow yard.
5) Underground transactions (illegal stuff)
The importance of cash will continue to decline with transactions with merchants, but it will never remotely approach "cashless".
And, as implied here, once we go cashless they've got us by the balls. People might not appreciate the value of easy, anonymous transactions until they're gone. Who are "they" you ask? Insert your favorite power structure. There is utility in being able to conduct private business without the need for any middle-man or money transfer service.
Remember what happened with Wikileaks donations and the credit card companies, when the latter started denying payment services for the former? Cash enables the average person to pay for or support things without permission. That's important if you want an empowered citizenry. I'll keep using cash, thanks.