I think what we will see is an arc similar to old World War I era aircraft. It will not entirely go away, but there will be a progressive concentration of operation into the hands of a few organizations and operators who make the effort to run and maintain steam locomotives.
At some point, I know that most engines will become unfeasible to rebuild if you want to keep anything authentic on them. Already we're seeing some rebuilds where it is an entirely new boiler and cab on rebuilt running gear. I think you'll see a group of replicas built that incorporate some modern methods to better handle FRA standards, preserving the experience of steam if not the original engine. I already know of one group on the West Coast that is planning a "modernized" replica to complement their current steam program.
I don't think they will go away, I just think they will get rarer. Operations like Cumbres and Durango, where the engines are such a part of the history, will likely keep them for a while, but the small groups with 1 engine or the "weekend warrior" organizations that spend 10 years rebuilding an engine to run on a shortline will likely disappear in a couple of decades, I think.