Some engines are simply too tired to turn another wheel. #191 ,RGS #74 and maybe RGS #42 are examples of that. #318 needs at least a couple of new corses in the boiler barrel to run. As Jason Midyette pointed out in the Colorado Timetable, several engines had restoration begun about the same time (2000), but, alas, only 315 made it as far as operating under steam. Us members of the DRHS resolved to restore the engine cosmetically if we found it too worn out to operate, or the funds simply dried up, A lot of factors worked in our favor, and happily we were not faced with that prospect.
Another concern is if too many heritage locomotives return to service, fans will lose interest in such projects. I maintain that such restorations should be staggered over time. The tough part is turning a critical eye towards historic power like #191 and make pragmatic decisions regarding the feasability of such restorations.
One human factor that is slightly funny is when the UP put Challenger 3985 into service, all the fans seem to talk about was "when will they put a Big Boy into service?" HELLO..! We should be grateful that an engines the size of 3985 is operating today.