Just because there's a crisis, there doesn't have to be a solution for there to be a good story. More amazingly, there doesn't have to be a solution for there to be a worthwhile ending. Maybe the survivors learn from their errors with the Doctor's help, or maybe he has to evacuate them in the TARDIS. Maybe what seems like the end of the world turns out to be the beginning of a new one. It's a new Doctor he can provide a new type of hope. Harlan Ellison was fond of saying of science fiction that even the darkest of stories were always hopeful because even in the worst situations, humanity has survived. Yes, humanity may have been enslaved by aliens or mutated by the toxins in the foods created by huge agribusinesses, but humans persist. Perhaps that's why the Doctor keeps talking about how indomitable the human race is, and maybe this Doctor will see some opportunities for humans to learn from their mistakes. Atop that, some of the best stories do have sadness in the endings. Earthshock, from the original series, comes to mind in particular; it featured the death of Adric, one of the Doctor's companions--and in this case, it was very much Adric himself who stopped the Cybermen. It's the possibility of failure that gives stories suspense. It could also give a reason for a return later, to correct what went wrong or to at least make things better for survivors. In an episodic show, after all, all options are open.
Michael Marcus is a game designer, author, and mad scientist living in Hamtramck, Michigan; his current project list include a series of comic short-stories collectively called "One-Punch," a book on hypnosis and language called "The Prometheus Codex," a collaborative game project called "Art War," and a fun spy story called "The Adventures of Jack Uzi" at http://tinyurl.com/JackUziChannel (for those interested).
