RE: Although subjectively downvoting posts to zero is anathema to me, the Layer 1 ability to do so must remain (for now at least)

I'm not sure I fully understand your question.

If someone revises their upvote, then the original upvote is removed and the new upvote value is applied.

For instance, if I upvote 100% then later change it to 0%, the author gets nothing and I get nothing. However, I do not get my upvote manna back, so zeroing out my upvote takes those author rewards away from the author and also takes those curation rewards away from myself.