These desisions have to be made, and will have to be made more in the future. There is an ever increasing population of preserved locos, yet an ever decreasing amount of spairs, and at the moment finance. When it comes to more modern locos, spairs become even harder as they are more complicated and less easy to just make. Something like a 56 will have a lot of electronics, you don't just bash them up in a workshop like you might other parts. What might look good on the outside can have all sorts of problems on the inside. A big mistake often made is to take something as a runner (or with some minor work a runner), clean it up, wack a coat of paint on it and call the job done, only then to realise how much work needs doing if it's going to survive long term. I know someone who bought a bus out of service, swapped a few panels and sent it off for painting. A year down the line they realised the floor was rotten and it's supports rusted, the air system needed a total overhaul, the gearbox was faulty, the electrics were a nightmare and finally when it went for MOT that it had 2 broken springs. It went for scrap.