“The effort has not been futile,” the voice of Paul Volcker roars out of the speakerphone, energised more by enthusiasm than anger. “The banks have eliminated their proprietary trading desks – they’re gone.”

The former Federal Reserve chairman is responding to recent criticism of the rule that bears his name, not only from well-established foes on Wall Street and in the Republican party, but also from senior Obama era regulatory appointees who have begun to publicly question the utility of the