I think we are all looking at it the wrong way.
We need to look at it Nigels way.
Nigel said : "If Brexit is a failure, I will leave the UK"
Nigel is still here, ergo (or as he would say "quod erat demonstradum, mein fuhrer") Brexit is not a failure.
(Of course that train of logic only works if you assume that success and failure are simple opposites with no shades in between. I suspect Nigels rebuttal would be that Brexit hasn't been the sort of success he was looking for.)