I don't want to start some longish economic argument here, but given that the EU is primarily a trade union and only around 1% of the total GDP of all member states is spent on the EU by its members, or, in other words, spending to the EU budget by member states is ridiculously small in comparison to national spendings by the respective national governments, it's hard to find an argument that could somehow contribute this development to the EU. How would more trade barriers improve the economy of individual member states? (Note that I do not deny that Germany and the UK might be able to exploit their strong economic forces in bilateral treaties to gain advantages over smaller countries within the EU in some trade wars, but I'm talking about the prosperity of the 28 EU countries as a whole.)
Moreover, if you honestly compare EU GDP development to the rest of the world, you will also find out that the EU has done fairly well despite the financial crisis that hit it harder than the US.[1]
The Euro has been a gold standard hung around the necks of the poorer Eurozone countries. They haven't been able to devalue, engage in Keynesian stimulus, or do anything else a country normally ought to do when confronted with a demand-driven depression, let alone a financial crisis.
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10 years of stagnation or decline of GDP in all top 5 countries of the eurozone (which account for 50% of all GDP of EU) is not a perceived issue.