I respect your position but I don't agree with your conclusions:
> That would lead to a ton of comments saying why they had flagged or downvoted things. Such comments tend to be of low quality
It's only low quality if the reasons for down voting are low quality and since the purpose of forced replies is to weed out low quality moderation, I think the ends justify the means.
> and (if people could reply) would lead to endless bickering.
People already bicker about the moderation. Constantly. What I'd hope to see it fewer people impulsively moderating posts. Which might have the ironic effect of reducing some of the complaints.
Obviously those who have a tendency to bicker will likely continue to do so but that will be little change to what they're already doing now.
> I once spoke to the creator of a much larger forum than HN, who told me the biggest mistake they made was to add a meta facility to the site.
That may also turn out to be true for HN but equally it may not. Every forum and community is different so what works in one place might not work in another -- and visa versa. As it is, the quality of comments have and peer moderation has already been on a downwards trajectory for a few years now so it's probably worth trying some new ideas to see if it does shake the less confident yet still experienced engineers back into contributing thoughtful posts.