Roger Altman, in the Washington Post:

Permanent stability will come only from full union across the board. And markets will support the simple currency structure only if they see a true plan for promptly achieving this. The 17 member-states must jointly put one forward.

One possibility: Eurozone nations won’t do this, and will suffer a horrific economic crash, and political instability unprecedented since the Second World War, including the upheavals of 1968. In a panic, nations agree to relinquish fiscal and political sovereignty for the sake of economic peace and stability.