I know you weren't talking about a break up of the EU, but my point is that the rise of the far right is not caused by EU membership in itself, and a situation where the EU did not exist would probably make it worse. A divided Europe would become more economically irrelevant and full of client states for either US/Russia/China competing amongst each other to attract investment.
It would also become more technologically backwards - one major reason the US is surpassing Europe economically is it basically has a monopoly on cloud services and most European enterprises are paying US tech companies (especially Amazon, Google, Microsoft) for cloud services and hosting. I think people underestimate how economically significant Google Cloud, AWS and Microsoft Azure are these days. E.g. Spotify is one of the rare European tech companies but it is all hosted on AWS so a large part of Spotify's expenses are paying US tech companies - no wonder Silicon Valley is awash with cash but Europe is relatively broke. A lot of the UK public sector expenditure will also be towards these kind of services, not sure how much exactly but it is significant.
On top of that Big Data also enables AI. China is the only country with a major cloud infrastructure not dependent on the US so that is why US and China are leading in AI.
The long term economic ramifications of this are significant - one reason why German car exports are starting to decline is because they are struggling to compete with American cars (Tesla) and Chinese cars which have connections to cloud services. There will be other effects further down the line.
Yanis Varoufakis argues that the problem is bigger than factory closures, as the automobile and green-energy sectors show.
www.project-syndicate.org
The fact is that the new technologies which will define the coming century are based on economies of scale. The EU remains ineffective and inadequate, but at least it is capable of attempting to do something. There is a thus far unsuccessful attempt to develop an EU cloud service - but it is highly unlikely that any individual European country would be able to develop their own cloud infrastructure.
The initiative, meant to boost Europe’s cloud services, is failing to fix the problems it was intended to solve.
www.politico.eu
IMO one reason why the left has failed despite false dawns like Syriza is that it isn't really possible for an anti-capitalist movement to succeed in one country which is integrated into the global market.
The USSR was only able to build an alternative system because it was an agrarian nation and barely part of the world economy so it effectively build an alternative industrial system from scratch, and also it was large enough and rich enough in resources to need little from global markets. I'm not saying the USSR should be a model but the integration of everything into the world economy today is also why even moderate social democratic policies we saw in the post war era are politically impossible today.
Syriza found their hands were tied once in power - but might it have been different if they controlled the ECB and had allies in power around Europe? I think of course it would have. This is why Varafoukis quit Syriza and tried to start his Democracy in Europe Movement, which seems to have basically fizzled out for various reasons, but I think it had the right idea.
So I largely agree with what you say about the left's failure to articulate a vision and being stuck as being against things, but left wing opposition to the EU strikes me as the most egregious example of this. I don't think it is possible to articulate a serious socialist vision today without it being an EU-wide movement.
Not saying the Green rejoin attitude is a serious socialist vision, but it is still something the left should support with the caveat that we want to reform the EU and that we can only transform the UK as part of a continent-wide transformation.