It is the nature of progress that what is now cutting-edge will, with the passing of time, become traditional. And it is the nature of human beings to remake and refine what has worked in the past, and call it new.
And so the term "traditional mystery" is from the outset somewhat difficult to define absolutely. It has an almost organic structure, with successive authors and generations adding their own extensions and renovations to the house built by the likes of Poe, Christie, James, Sayers and Conan Doyle.