Theory: Hitler & Nazi Germany are merely "maximal evil" words for most of modern Western secular culture. If you went back to half-millennia-ago Europe, then something fairly similar to Godwin's Law would apply to Satan & Hell. Similar for other historical periods and cultures.
The conversation-killer isn't the application of a reference to Hitlerism. It's the invocation of Godwin's law to squelch any comparison to Hitlerism.
The fact that the probability doesn't approach 1 implies that the number of conversations in which some participants veer toward Hitlerism (1) is greater than the number in which interlocutors are willing to rise to the bait. Probably because they heard about "Godwin's law", the most effective tool since holocaust denial to silence people who will call out fascism.
OTOH, Western culture has been strongly dualist (good/evil - https://en.wikipedia.org/wiki/Dualistic_cosmology#Moral_dual... ) for the past 1500-ish years. In non-dualist cultures, things might work quite differently.