Considering we lose a lot from the fee's to be in the EU, the lack of control we get over our own laws, the health tourism, the welfare system the EU wants to let romanians and others abuse, the freedom of movement that lets any low rent worker in etc. then I think the economic argument wouldn't be as strong as it's made out to be if you factored it all in. Add in the forced migration of refugees germany accepted (it'll happen eventually) and the expansion into allowing turkey in and we'll see a lot more people wanting to jump into our country too.
We could always form a common wealth union that wouldn't want to tie us down to crap like romania, turkey and open borders too

Also if you expect people to concede an economic argument then you have to be smart enough to concede points against the EU too like the expansionism and lack of sovereignity under it. You might not particularly value that point personally but it doesn't change it's impact just like raising the minimum wage would have no benefit for someone paid more but would still be of benefit to many, you seem to undervalue having control in your own court. Most countries are better served being able to use policy that matches there own needs in both value, speed and effect along with adding any concessions for country specific reasons. The EU just is a one glove fits all policy, do you think most companies would survive if they had to negotiate, concede ground and manage a business based on general vote with 28 other businesses? It'd cause issues to say the least, they should be using smart management to move around and make changes they need but the EU is the opposite of that approach so the economic hindrances it places. I doubt there's huge issues with how it's all implemented but even just a few uncompetitive and hindering regulations would add up for an entire economy that should be making it's own decisions rather than being forced along with general stuff.