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

[logiczombie]

In the original system, your UPVOTE bar could be used up by UPVOTING (OR) by DOWNVOTING.

You could choose to DOWNVOTE but that would reduce your total available UPVOTE.

After HF21, a FREE DOWNVOTE bar was added, so you could DOWNVOTE with NO PENALTY.