Worth engaging on that point, as I think the crux of the disagreement is about boundaries on rhetoric, and whether downvotes are an appropriate enforcement mechanism for a kind of rhetorical social contract. (pls correct if doesn't capture intent)
A billboard (or search results) with false accusations are a good example, as it exploits others' limited ability to reason about them, sort of like propaganda. It's why there are laws about libel, "fighting words," incitement, false advertising, harassment, perjury, and I'm sure others that fall outside free speech guarantees. They aren't great, or even very good, but they are examples of existing legal boundaries on rhetoric, so the "all/any discourse" already has some limits. Then we've got logical fallacies, and civility etc, so these are checks and balances in a dynamic and narrows the scope away from extreme examples.
Regarding "any discourse" being necessarily better than none, I'd very reluctlantly accept that position, but I would certainly say "most, and almost all discourse" is better than none. All and here? No. Most and here (public)? I'd still err on the side of liberality, even though I'd agree there exist a bunch of common systematic tactics designed to destroy discussions, but public discourse has evolved to handle an increasing number of those tacitcs by recognizing them as trolling.
My position is that discourse is hard, it takes practice, and when it's done well, it is a very refined human art that is an honest signal of thoughtful work. Even the "Dumb." comment in the example, if it were comedically timed or in context it might be brilliant, and is way better than a downvote. When discourse is crappy, it also signals the quality of the thought that goes into it, and I think that's a feature. Nobody is born a great thinker, and the only way we get better and create value we can share with others is by testing our ideas, with opposition. I'm saying downvotes subvert and even pervert that by rewarding spoilers.
That said, I'm oddly in favour of flagging as a tool for things completely out of contextual bounds because that's a request for refereeing, but absolutely against flagging on a global scale.
Regarding the busker, I'm saying that if to get a busking permit in a square you had to have a little box people could hit as they walked by that tallied peoples hostility toward you, is just as absurd on the internet as it sounds in real life.
Pleasure to engage! :)