Plausible deniability all over again. If you don't reply, it can always be seen as if you've forgot/left on vacation/got eaten by a giant squid. If you do reply, you signal with the quality of your reply, and so if you don't do your best in the context of the conversation at that point, it's a negative signal based on what's expected. 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. Deciding to reply carries a sentence of having to reply well and also to probably have to reply to the follow-ups. Not replying at all is the only way out.

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). This is somewhat in conflict with mental hygiene: you shouldn't normally expose yourself to statements you can't sufficiently verify for yourself, but for this particular problem the balance seems to be in the other direction.