The EU started out as not even a half-hearted attempt to make the continent one big, efficient nation.
If Britain leaves it wont impact the world that much. Their economy would shrink a little immediately, followed by expansion. The question is whether all that would be better than the so-so expansion they can expect with the EU.
In the long run, i dont think it matters if brexit happens or not. The real problem is how pathetic the EU plan has been executed. Its almost as bad as Russia's transition to capitalism. Eventually, maybe when im old, the continent will be one big nation. The fact that countries occasionally consider leaving means there is little patriotism towards being in the EU. It doesnt feel natural to the people. A state in the US has not considered leaving our union in about 150 years and that was mainly due to british instigation. The us has every state represented in the government and federal laws are pretty uniform. Europeans dont want to be one big country yet. They just want a better deal. They do not understand how truly joining together is better. It will take a couple generations before random europeans grow up considering themselves European first, and german or italian second. There is no real pride yet, just a deal that kinda sounds better, hopefully, maybe.
Eventually random Scandinavians and poles will tell people they live in the EU and the state they grew up in was norway, poland or Romania. That is when there economy will be doing well. It will be awhile. For now, Britain should do whatever helps them sleep at night.