As you are so certain that no such person is ever likely to exist, your question is hypothetical. Any answer I gave would therefore be hypothetical too, so there's no point in wasting my time or yours.So if, for example, a leading industrialist who had campaigned for Remain now announced that the Remain argument was based on lies, and he now felt Brexit was a fine opportunity for the country, you would refuse to listen to him on the basis that he was a turncoat?
I'm not saying that such a person is ever likely to exist, but I would like to test the consistency of your views.