Nevertheless, some people find them useful.
I don’t think it’s 100% clear-cut that you should never ever implement downvotes. I’d say that downvotes need to be implemented in conjunction with a bunch of other features and active moderation policies to work well. I don’t know if HN has done so, or if GitHub has done so. But... I wouldn’t dismiss them out of hand.
(FWIW, I follow this policy: I either downvote, or reply, but never both. In this case, I disagree with you, but think the conversation is useful, so I reply. Other comments might be wildly off-topic or deeply non-constructive, and those I down-vote. But to each their own.)