At that point the countries of the South (and North? Ireland..?) will keep devaluing daily, while the markets will push Germany's new currency through the roof, turning exports virtually impossible. Unemployment will struck Germany and all hell will broke loose. Repercussions for the South will be devastating of course.
That's not my preview. That's the scenario Varoufakis foresees and probably the author too. Makes a lot of sense to me, but Varoufakis might be wrong. Time will tell I guess.
Just keep in mind that Greece, Italy and Spain are in dire straits anyway so the prospect of defaulting doesn't seem all that scary.