One of the recurring themes in Doctor Who is that all of time and space, the entire history of the entire universe, is big. I read somewhere, for instance, that the fairly light adventure "Planet of the Dead" must have actually happened several months after the insanely dark "Children of Earth."
Essentially, no matter how miserable things get at any given point in history, the universe moves on. Both a depressing and reassuring message. ("This too shall pass.")