Have you ever experienced a natural disaster? One thing you realize quite quickly is that your debit card is useless. That doesn’t mean transactions stop, people are still buying and selling, those with cash at least. If there’s runs on banks, infrastructure outages, etc, those relying entirely on debit and other payment cards won’t be buying food!
That's like $1.2 trillion in boring, normal, law-abiding cash transactions each year.
Also, for the record, I have been in and responded to a number of natural disasters in the "first world". With the exception of a few hours here and there, using cards was never a problem. If the infrastructure is so wrecked that cards don't work, food/water is coming from people in high-vis vests.
Try a hurricane, or something serious enough to be registered as a natural disaster.
The fact that lots of people in India have had serious issues because they were unbanked does seem to indicate that there's a sizable population of people using cash for everything*. I'm curious if there's any research into whether that's for tax-evasion reasons or for some other reason.
My personal belief is that the number one use is for money laundering, and that people who are using it that way would be willing to pay taxes on the money they are laundering from some other illegal activity. For instance, there are all-cash drug dealers who pay taxes on their drug money. I'm sure Al Capone wishes in retrospect that he'd paid taxes...