There's no reason to doubt that leaving the EU will have a downward pull on UK economic growth, but it'll be hard to see in year-to-year fluctuations and a lot will depend on how the UK uses its limited but real increase in freedom of manouvre.
The EU is not that dynamic as an economic entity. At its worst it is a neo-liberal strait-jacket. I expect Britain leaving will work to its benefit in the medium term, making for example the CAP much less egregious and shifting gradually towards an integration of social spending.
The supranational level of the EU is strikingly undemocratic, but subsidiarty is substantial and I've yet to see a clear-cut example of the EU preventing the UK from having done something it would have liked to (IT regulation might be or become an exception).
There certainly isn't an EU demos, though ironically the Brexit saga has probably moved the European population a few degrees closer to a supra-national 'imagined community'.
The most concerning to me was the prospect of the Irish of Northern Ireland finding themselves locked tighter in an alien nation-state. Europe was a sphere of sovereignty that cracked a window on the 'British state for a British people'.
Irish diplomacy avoided the real risk of a re-partition of Ireland. If anything, the door of the Union has now been pushed further ajar without, I think, doing too much violence to the dignified British sensibilities of NI unionists.
The most depresing thing about Brexit, other than it having been the victory of a right-wing project, is the insularity of it. This is probably more evident to someone who's Irish. I'm of a generation that remembers FSL Lyon's famous reflection on southern Ireland during WWII.
"It was as if an entire people had been condemned to live in Plato's cave, with their backs to the fire of life and deriving their only knowledge of what went on outside from the flickering shadows thrown on the wall before their eyes ...
by the men and women who passed to and fro behind them. When after six years they emerged, dazzled, from the cave into the light, it was a new and vastly different world.” ('Ireland since the Famine', 1971).
Truth is, there was something of Plato's cave to independent Ireland for decades. Joining the EEC was a watershed for Ireland as it plunged back into the Old World maelstrom, with all its frustrations and excitements. It made Ireland independent *in* the world.
England's dreaming, now, will not be comely maidens dancing at crossrads, but warm beer, cycling vicars, cricket on the village green, Spitfires and poppies, 'white heat' and the Iron Lady. It's a psychological retreat, snuggling into a comfort blanket by a fire in the cave.
The post-war literature of British dystopia ('Day of the Triffids' and such) was disparagingly described by Brian Aldiss as "cosy catastrophe". We'll find that Brexit is no catastrophe, but it'll be all too cosy. Not an end to history, but a burrowing into it.