Again, the European countries has been wealthy before the EU, so we can say that at the moment (due to the lack of central taxes and treasury) the situation is worst than if they are independent.
And there is no relationship between country size and wealthiness ,by your logic the small countries are poor, the big are wealthy.
Maybe because the success is how effectively a group of people using the available natural/human resources, and compared to that the cost of transactions is minor.
I'm not saying that poorer countries don't benefit from joining the EU.
But the EU is not a true single market when you compare it against the USA or China for example.
There are structural issues such as language that will stop it becoming a truly integrated single market.
So there are lots of places in the EU which can't use their available natural/human resources, because they are trapped in their own market.
Look at the horrendous youth unemployment rates and low wages in Southern/Eastern Europe, which have driven mass migration towards Northern Europe. And that migration has already contributed to a huge increase in electoral anti-foreigner sentiment.
Yet so many more people are still trapped in Southern/Eastern Europe, because they can't speak any other languages.
Labour mobility is one of the key elements of a functioning single market.
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And in terms of freight, where are the trans-european railway services or logistics networks?
And what about re-imposition of border controls between countries inside the Schenghen Zone?
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Like I said before, countries joining the EU is generally a good thing.
But it labours under structural issues that will stop integration.
English is the defacto language of EU politics and business, yet only 15% of the population have English as a native language.
Can you imagine the European Union imposing English on all the member states?
Can you imagine France, Germany, Italy, Spain etc etc, all agreeing that their existing languages are useless?
And that all schools, businesses, government offices, TVs, road signs, etc etc must be replaced with English?
But a single national language is required in order to forge a single political consensus with a strong central government that has budgetary power.
And a single national language would eventually mean a single market for labour, but this would be a project for generations to come.