Two thoughts. In the original series of stories Conan Doyle killed off SH at the Reichenbach Falls, but there was such a public outcry for more that he brought him back and wrote more stories. Secondly, the original stories were written in the person John Watson as a sort of memoir (like in this series JW is writing a blog), so there is no reason for this to be a final one as JW could produce his memoirs in any order and give new stories at any time. Anthony Horowitz is writing new stories now.