...and now you've found the problem at the heart of the European Union.
There's a lot of disagreement about this. But fundamentally, the EU is trying economically to be one country while legislatively it's a whole bunch of member states. And while, say, the United States of America can take money from California to pay for welfare programs in Alabama, the EU cannot take money from France to pay for welfare programs in Sardinia. More than that, Sardinia can pass laws about who gets welfare and nobody in France can say boo about it... but if Sardinia passes laws about price floors on tuna the EU can overrule them and shut 'em down. And by "the EU" we mean "unelected bureaucrats in Brussels that are de facto immune from recall or election by the people they govern."
It could all work out. It could continue to be groovy. I don't know if you've seen the James Bond movie Goldfinger, but the core premise is an evil billionaire tries to irradiate all the gold held at Fort Knox, thereby radically increasing the value of gold, impoverishing the United States, making himself crazy rich because of all the gold he owns and plunging the world into chaos as there is no longer any backing behind the world's currency. Goldfinger came out in 1964. In 1971 the United States said that the dollar was no longer convertible to gold, effectively doing everything Auric Goldfinger intended without nukes or spies... and this is the first time you've heard of it.
So we can make all sorts of dire prognostications. We can make all sorts of utopian predictions. But nobody knows what happens if somebody leaves the Euro. Current odds are as soon as one country leaves everyone else will be gone by sundown and then who knows what happens... but it's not like we have experience with this.