RWBY - It really was a mistake

Nobody wants RWBY to get entirely dark. They love a light-hearted stuff. They still want the light-hearted stuff. But if it is like that 24/7, then it loses meaning/impact.

By giving the story/plot a more serious tone; you are now concerned about the well-being of these light-hearted characters you have now become attached to. The drama created by this setup is what people are looking for.

It's about creating the right balance between fun characters and a serious story.

Not the amateur's understanding of "adult and hardcore", but the jaded kind.