Europe's most successful countries are the ones with their own currencies. It's easy to compare the differences in growth and debt with those in and out of the EZ. Ireland is the only true exception & it's for reasons relating to its tax policies.
Germany and Benelux are high but problematic in areas of consumption. 41% of the German Economy is export orientated (meaning consumption is extremely low) and while employment rates are high, employment isn't completely 'full time', wages lag productivity significantly (pay is relatively bad) and debts in the private sector are heavily problematic. Not to mention Germany's population demographics don't have a bright future.
Contrast this to economies with their own currencies - the UK, Sweden, Norway, Poland, Denmark - which aren't dogged down by such problems. Sweden had a housing crash last year and is managing just fine despite it.
If the EZ had another crash it would be debatable whether it can still hold strong - a point reinforced by France's Finance Minister himself.
If you look up GDP it is pretty big, but if you look up wages you will find its relatively bad.
But don't over exaggerate, we are just about to see the story when one tries to play on their own without the EU. I think it's going to be the first true example of "what happens"
It's very distracting to keep looking at the UK and Brexit and forget that others on the continent clearly do better without the Euro. There are still countries in the Eu with EZ opt outs.
I'd also like to point out Switzerland is not in the Euro and not in the EU and its metrics are far better on everything - although it is in Schengen.