Scott Adams imagines one:
When you eliminate cash, you also eliminate a lot of crime. Criminals need cash to stay off the radar. In a cashless world, drug dealers and crime syndicates could try to set up fake businesses to launder their revenues, but it wouldn't work. Imagine setting up a fake dry cleaner, for example. The government could easily determine whether that business is buying the type and quantity of dry cleaning supplies typically needed, and whether the profit margins are at industry norms. All of that information would be available through the tax records. A drug dealer could pretend to be a consultant, but even then you expect a digital trail for buying printer ink, business travel, and the like. Perhaps the drug dealer's address and educational level would be tip-offs too.