That applies to each and every decission. If we accept this argument, policy discussions are meaningless.

Let's be less theoretical: if Britain does much better than the EU it could be due to:

1) EU was bad for UK

2) The UK is abusing its new position (tax heaven, ...)

3) ??

If the UK does much worse than EU, this means:

1) EU was good for UK

2) EU is bullying UK

3) ??

So we'll be able to see what is happening.