Discord in no way "kills" mono decks - it just reduces the amount of quanta available. Mono-decks usually have quite a high pillar-to-other card ratio, perhaps the highest in the game. Aflatoxin doesn't "kill" decks that rely on bone walls, either. It just makes stuff harder. Anyways, what decks "rely" on bone walls, anyways? Just a quick Plague and a Graveyard can quickly turn the tide.
However, this card literally destroys any splash deck, and there are a lot of great splash decks out there.