By ordering the banks to close their accounts. People are not holding significant portions of their wealth in physical cash outside of the banking system. Apart from it being absurdly dangerous, there's also not anything like enough physical currency to do that. Not to mention investment accounts, deeds to property, etc.

Businesses being forced to accept all the risk and inconvenience of accepting cash to protect people from having their assets is somewhere between a security theater and a straight up pipe dream.