During the history of photography, there has arguably been no component which has improved as much as the medium which records the light. The mainstream has gradually moved from glass plates, to 5x4" "press" cameras, to TLRs (120 film with 6x6cm images) to 35mm film (24x36mm image) and finally to sensors with a wide variety of shapes and sizes.
At the same time, there is obviously some metric which decides whether a particular image is good enough to use for, for example, newspaper reportage. For the sake of this question please could we keep it simple and assume that it this largely based on the resolution of the image (line-pairs per inch, pixels across the diagonal etc.).
Is there any established rule of thumb which suggests that the resolution offered by a TLR, the resolution offered by a 35mm camera and the resolution offered by a digital camera were roughly equivalent when they started being favoured over their predecessors by e.g. photographers accompanying journalists?
Although this question might sound hopelessly naive, please assume that I am familiar with both sensor and film technology (halide grain structure, the possibility of multiple active sites per grain etc.). This was initially going to be a simpler question asking at what point (megapixels per sensor) journalists started considering DSLRs for their work.