Almost no single policy changes can ever be shown to fail or succeed so clearly as this. So many other things are going on at the same time. Those arguing in bad faith [which is a large proportion of those arguing] can use those things to muddy the waters.
If Brexit seems to founder, Brexiters can make bad faith arguments about counterfactual Brexits that would have been great were it not for the bunglers. In fact, this process has of course already begun!
It's just this problem that motivates social sciences like economics. Here we use natural pseudo experiments afforded by history - see the econometric analysis of the impact of trade barriers - to see whether Brexit will 'fail'.
The disadvantage of this approach is that we can't tell how the general case of trade barrier erection applies to our UK case. But the advantage is we already know the answer even before we have embarked on it!
David's projection of how Brexit and its diagnosis, and the effects of that diagnosis, play out also fail for other reasons. He presumes that the point of Brexit is still to have particular political and economic effects. But things have got so ridiculous that this may not hold
A bit like the French revolution, which became ever more extreme and self-consuming, Brexit - caricatured, and in the minds of some, only - has become a moniker for tribal warfare that is already on that same path of mission redefinition.
Brexit can't possibly be seen to fail, if this is indeed what is happening, or will happen. And there is ample mud in the social scientific structure of it, for a failure not to be discerned as such.
And the knowledge transmission mechanism - borrowing @sjwrenlewis ' term - won't guarantee to correct us either, as enough of the media will bend to accommodate the tribal messaging and factlessness.
