The Doctor is not a hero on a quest he is a catalyst. Because of this he is not limited to the protagonists journey with its defined beginning, middle and end but may instead drop in where he is needed becoming the force for change. He is the fly in the ointment, the nail in the tire, the wrench dropped into the gears of the machine. This is why his story can be told over and over again and his situation constantly reinvented while still making sense. Doctor Who works in both past and present forms because the character of the Doctor is a flawed individual who isnt always interested in saving everyone or anyone. He exists to upset the apple cart, to move things along in a new direction and sacrifice is part of that. The Doctor is not a savior he is a destroyer of the status quo.