Keep in mind that it costs VP to downvote (and hide comments). If that comment was upvoted by ~$0.05 then it would be visible again. The downvoters of that comment only did so with a rather small downvote. A 100% upvote by anyone with 3000-4000 SP would overide the downvote, and make the comment have a positive payout. You can rent that much SP from DLease for around $5 a week. With that you could undo ~50 of the comments hidden by this downvoter per week. Of course they might fight back and downvote more, but IDK.

This system was mainly designed to counter spam, plagiarism, etc., not to hide legit comments, BTW. Adding in a settings option to Steemit to show hidden comments be default would probably be a good idea: I might add that feature in a bit. (by writing a PR)