Any medium dies (or falls into relative obscurity) when artists stop using it and this will happen when artists fail to get any opportunity to show their work in that medium, or to see other artists' work in that medium.
This past year I have seen more shows in Boston area museums of traditional photography than in any year I can remember. In the gallery I run I have had more traditional photographers ask for shows than in any year previous to this one. Our B&W photo classes are filledl every year.
Even relative obscurity is a fruitful condition for many mediums. I give you Bromoil and wet-plate photography as examples. They are not seen in the numbers that digital prints are, but good examples of images produced in these methods are often strikingly beautiful to viewers who have no understanding of the medium at all.
I'm reminded of roteagues's reference to an unthinking comment in another thread - to the effect that color photography was just colorful calendar art. Colorful calendar art will always have a place in the home. It is an inexpensive approximation of an artwork. The real artwork is an original, not a reproduction reduced in quality and presence for the sake of economy and popular distribution. In a way, we can thrive as artists because of this bastardization of our work. Any magazine shot, any poster, any web use of our photo is "calendar art." Thank god for it, it pays the bills and lets lots of people see what we can do. It may even inspire other artists. Only a fool considers the (digital) reproduction the artwork.