A sloppy reply or a "this is what I believe if not why and I won't continue this conversation" will even signal that you don't respect your interlocutor enough to give a proper response and explain your disagreement, an inference worse than if you didn't reply at all.

I tend to non-reply for this reason often.

We need a norm not for stating your last position, which is a burden on the person who has to reply, but for accepting irresponsible declarations, as last words in a conversation (and some way of signaling that this comment is in the "last word" mode).

So I appreciate this insight.