Valuable conversations can extend from comments that are already negative.
If a reply to a downvoted comment is not downvoted, replies to that reply are not punished, so good subthreads are unaffected.
-3 is also not that negative.
5 Karma points is not that much as well, so if it's really worth replying to, it's possible to continue the conversation. It's usually not worth replying though, and when not feeling any cost people would ignore that consideration, giving fuel to bad conversations as a result. The motivation I agree with is to stop bad conversations, not necessarily replies to individual bad comments, which is just the means.
Has anyone yet mentioned or reported that for the last couple days, the "karma for last 30 days" is showing zero for everyone? And that we no longer can see the top contributors for the last 30 days either?
Do we have an explanation or estimation for a bugfix on this?