I have not heard of Godwin's Law implying that (useful?) conversation after the comparison ceases, but rather simply as: "As an online discussion grows longer,

I have not heard of Godwin's Law implying that (useful?) conversation after the comparison ceases, but rather simply as: "As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches 1". I've never liked Godwin's Law much, because it's trivial. Another way of stating the identical concept is: "it is not impossible to make a comparison involving Nazis or Hitler". The version you describe would at least have some non-trivial component to it, but e.g. on its Wikipedia page I don't see any reference to usefulness of subsequent conversation.