That's a common misconception. Downvoting for disagreement has always been ok on HN: https://news.ycombinator.com/item?id=16131314
I think people have the wrong idea about HN downvotes because they think Reddit rules apply to HN. It's a bit like how in Canada we think we have Miranda rights because we've seen it on American TV.
The problem with your argument is that it doesn't reckon with just how lousy bad internet comments are, or how many of them there are, or how completely they take over if allowed to. To a first approximation, bad internet comments are the entire problem of this site.
It's easy to imagine an HN that would be just like the current HN, only with some negative X (e.g. bad downvotes) removed. Most of these ideas are fantasies, because the thing you'd have to do to remove X would have massive side effects. You can't hold the rest of the site constant and do that.
Bad != disagree. I think we all agree that it should be ok to downvote and hide "bad" comments. However the problem is that many good comments are downvoted simply because people disagree with them.
I think it might be better to remove the downvote and replace it with "flag", so people can flag bad comments (spam, abusive, pointless, etc). At least that way people would need to think a little before the comment gets flagged, which would hopefully result in fewer minority viewpoint comments getting hidden.
I think you have more success if you ask everyone else to upvote unfairly downvoted comments.