I would think the Silverton was saved by the improved transportation system in USA. Remember that automobiles were hardly available during war years and supply was pretty limited until about 1949. In addition from vacations to Colorado in the late 1940's I can tell you there were a whole lot of unpaved roads in that state and some of the roads were just awful. In the 1950's improvements in the roads became common place and tourism started a big up swing. Once the "secret" of the trememdous scenery got out it became Katy-bar-the-door on abandoning the Silverton route.
My .02 cents worth.