While it would have been great for most of the K-28 class to have survived to modern day, it may be worth pondering some of a few considerations DRGW might have had.
The locomotives would have been well worn from their stint in the Army. Considering that all the railroads had to catch up on deferred maintenance after the war, the K-28's would have been just something else that had to be rebuilt. In addition, DRGW would have had the extra expense of buying (they would have had a value as scrap) and transporting them back "home". Why not rebuild something on hand that is already paid for?
Does anyone have a list of DRGW ng assignments for 1945-46? It is very possible that they simply did not need these locomotives any more. The K-36's and K-37's would have been preferred for their greater pulling power and resulting lower labor costs. Where would the K-28s been used that couldn't use a K-36/37? (Silverton branch for one). Weren't there enough K-27s left to handle those same assignements (with the remaining K-28s)?
DRGW might have seen the handwriting on the wall regarding the ng back then. The network of roads would expand as fuel and new vehicles again became available. Many returning veterans had experience as truck drivers and mechanics, this caused a huge acceleration of the trucking industry along with the established production lines for trucks. What ng branches were standard gauged after the war and continue to survive today? (Even the Monarch is gone).
It might have been as simple as "Accounting said no!"