In the context of my post, how the other 27 are doing is almost irrelevant. If we want to (truly, accurately) know how the UK would have fared had it remained, the only way to know this is to assess a parallel universe EU member UK that never left, with all other variables remaining the same.
This is why these discussions become cyclical, there's no definitive way to conclude one way or the other.
You have changed the question in an attempt to prove that it is impossible to say if the idea was successful.