I doubt cash will be outlawed, though. It will just be more and more inconvenient to use it outside the black market. A stigma will develop. Perhaps use of cash will be considered suspicious, and result in police detaining and questioning people more. Merchants will call for a manager to handle cash transactions as a matter of business policy. The average Joe will contentedly use a credit card for everything. And all the while, "legal tender" laws will go entirely unchanged.