It seems that fire prevention is the main reason for converting steamers to oil firing and buying diesels. But there is also reference to the increasing difficulty of maintaining the steam locomotives indefinitely. In reading the extensive D&S article on TRAINS website, I get the impression that while D&S plans to operate indefinitely, their use of steam is expected to end at some point due to the difficulty of keeping it running or replacing it.
But as long as there is a market to be in the steam business, why should it end just because things wear out?
It is true that new diesel locomotives are available as a production manufactured product, whereas new steam locomotives are not. Instead, a new steam locomotive would have to be custom built. However new diesels for 3-foot-gage are not readily available as production manufactured products either. So why is it more difficult to run steam indefinitely than to run diesels indefinitely?