I dislike this mentality. It's fine to "reuse" art assets if it makes sense (like the dungeons are part of a group, similar environment and a closely related story), as long as the game play is different enough to be engaging. In fact, not reusing assets is likely the cause for reduced amount of content/dungeons, since they have to focus on putting out more art to appease this ideal.
But then, more content is better than more art, to me.