It doesn't have anything to do with "turning toxic". It's only natural to think of a downvote as the opposite of an upvote. So, if upvote = "I like this", then downvote = "I don't like this"...whatever the reason is. It takes a lot of work to change something like that, and even then you'll get newbies doing it wrong. It's a lot easier to just change the interface.

The real key, I think, is to have reports seen by a human. It's easy to abuse downvotes, because there's no accountability for it. When you report something, you know that a mod or admin is going to look at it, so it's not very likely you're going to report a comment just because you didn't like an opinion.