One downside of critical comments is they tend to draw attention to the discussion. Mass downvoting suggests that something is so low quality you don’t have to pay attention to it.
Yeah, in case of obvious crap posts (like spams) they’ll be massively downvoted. Otherwise, I’ve never seen here any of the serious posts massively only downvoted. Rather, you’d have some downvotes, some upvotes, and the case you describe doesn’t capture this situation. In fact, an initial row of downvotes may misleadingly give such an impression, leading to some people ignoring the issue, while later on a row of upvotes may actually show the issue is controversial, and as such indeed deserves further discussion.