I wonder how much of this had to do with the shortage of strategic metals during World War 2 and the sudden need for them and the expansion of the economy into the 1950's? After all, those shiny new diesels had to be made of steel, and there was a tremendous amount of scrap steam locomotives just waiting to be cut up for the smelter. There was a huge expansion in consumer goods from the end of WW2 for the next couple of decades, and plastic wasn't really a "thing" yet. When you think about it, there really aren't an awful lot of 1st Generation diesels that survive. Considering the huge rosters of E/F units and early GP/SD's, there aren't many still around.
As far as Denver not receiving any steam locomotives for display what about 4005? Was it donated to the city, or directly to the museum?
Robert