I get you, altho that I'd just report.

If we took a less spammy/insane thing for example though, like just a low quality post, I understand that. Not objectionable enough to report, and no utility in responding because it's just noise and you'd just increase the noise. And there is utility in getting rid of it (or at least deemphasizing it) because it gives space to actual discussions.

I think that's a reasonable use case for downvotes. It's definitely not the only way they are used (e.g. disagree button), but valid anyway. There are other mechanisms to sort this way too (e.g. number of likes + age of comment, so old comments with no/few likes sink - new junk still sits at the surface for a bit tho, possibly longer than if downvotes could drop em). But worth more thought for sure.