Downvotes are useful to make bad content sink. Without them, the bad content has the exact same score as fresh new content, content that failed the Fluff Principle, etc

I don't see how downvotes help filter content. It makes sense at first, but either people are sorting content by New, in which case votes do not matter, or they are sorting by Top and will get only the "good" content. Several instances already have downvotes disabled. I don't see any complaints from their users about "bad" content having the same scores as "good" content.

lemmynsfw had to disable downvotes because gay content posted in gay communities was being downvoted. It wasn't being downvoted for quality, but for not being what the majority of users wanted to see. That doesn't mean all users now have to see gay content they don't like because they can't downvote it. It's still easy to filter using the block feature. Again, I've never seen users there complaining about being unable to filter good from bad because they can't downvote.

if you don't reduce its visibility, some clueless muppet is bound to interact with it, usually generating more bad content.

I've seen posts and comments with -100 votes often get lots of interaction from people who can't stop themselves from arguing with a troll. Sometimes there's only 1 or 2 comments under a post so the score doesn't even change its visibility at all.

Either way, giving people way to say "I disagree!" without interfering on the main purpose of the button - sorting content.

The way to say "I disagree!" is with the reply button! Votes don't prove who is right and who is wrong. I've never changed my opinion because of downvotes. Sometimes I even agree with a downvoted comment because I form my opinion based on arguments, not votes.

I also like seeing different opinions. Yours gave me a lot to think about! It'd be a shame if people didn't post their thoughts because they feared being downvoted for it.