But how do you draw the line between garbage content and quality content?
By your own subjective standards. We do not need to have totalitarian rules on this, the consensus emerges from the individual and independent votes of all interested stakeholders.
What you advocate for in essence leads to censorship.
Not so. Removing some potential financial rewards by down voting is not censorship. It is showing the market value of the post to be less than it was a previous point in time when the opinion of another stakeholder is factored in.
You make a valid point. Perhaps "censorship" was a bit extreme. I'm not against the idea of down voting, just against a systematic pattern with no objective rules.
The biggest issue I have is that timcliff brought this up as a way to combat abuse by those who are self voting useless content. It isn't a solution to that problem. Logically, when their posts start getting down voted, they'll do the same to others. And then it becomes a measure of who has more vests.
Perhaps down voting that only impacts a post when it has more downvotes than upvotes, or is above a certain threshold would work better.