I am going out on a limb here and will cite the Nickle Plate as one example of what happened. They had a fine fleet of modern 2-8-4 Berkshires totally suitable to their operation. They were able to operate them at an operating cost which was competitive with diesels. However the steam locomotive is a labor intensive machine. Any group operating one today will confirm this. The cost of labor increased rapidly in the 1950s and this was likely the single greatest factor in the demise of NKP steam in July, 1958. Another contributing factor was the increasing difficulty and expense in obtaining new and rebuilt parts such as air pumps, feedwater heaters and injectors, driver tires, etc.

Regarding "todays technology" contributing to successful contemporary steam; perhaps Ross Rowland would be most qualified to comment. He accomplished more than anyone else in this area.