You are rejecting my point simply because you don’t like the conclusion.
No, I'm trying to get you to understand how your point is simply irrelevant to the discussion.
You are determined to press your case fixating on the MEANING of a Replacement Proverb, which we cannot understand because it’s an ancient proverb fossilized and repeated until nobody knows what it means, but they know when to say it, like responding to a surprising revelation in English with, “How about that!”, which uses a combination of words that make no sense together whatsoever, but we all know “what it means” because we’ve witnessed so many people utter it under similar circumstances.
Let's use your analogy: <out of habit/courtesy or “Ga-Zoon-Height”, even if they don’t know German. This might be like that.>
Let's say that we don't know German, but we're told that "Ga-Zoon-Height" means "to your health". We might ask, "How does Ga-Zoon-Height" mean "to your health"? "Does Height mean health in German?", etc. Given a sentence and its translation between English and another language, we can ask: "how does this sentence have this translation?" It does not matter the original reason why German-speakers say the word "Gesundheit" when someone sneezes.
Imagine that {reH latlh qabDaq qul tuj law' Hoch tuj puS} came up in a regular conversation, and wasn't being used as a replacement proverb. We're told its translation is "The fire is always hotter on someone else's face." Why does it mean that? That's a question that can be asked, independently of the fact that that sentence happens to be a replacement proverb.
If you're uninterested in the grammar of the sentence, that's one thing. But the fact that a sentence is a Replacement Proverb doesn't make its *grammar* impervious to analysis. (Its *meaning* may be lost to time, but that's a different thing.)
Explain what “How about that!” means in English, breaking down the grammar and explaining the choice of each word in relation to the meaning of the sentence.
This isn't an analogous situation at all.