It takes a long time to design and a lot of material to make a big transformer and they last for decades. They are critical infrastructure so when one fails you want it replaced or repaired quickly but they are too expensive (millions of dollars/pounds/euros) to keep a spare unit idle for decades and the lead time for a new large power transformer might be six months or even longer. So repair makes sense, especially if it can be done on site because moving a big metal box full of oil, steel, and copper that weighs hundreds of tonnes is a major logistical exercise.
This isn't something that can be done by a company that 'moves fast and breaks things' and disappears next year.
There must be some extremely salty machinists that get bizarre part requests after breakage for some transformer from 1950. That could make a very cool YouTube video series: Following these machinists and repair people as their repair very old transformers.
But of course many such transformers are connected to equally old generators and they are a bit more challenging as far as making replacement parts is concerned, huge turbines for instance.
Then define the version control system and the build process, specifying the dependencies, and so on.
Think about any opaque blobs in the system and try to eliminate them so that you have plain text source code so that no tools are needed to read the code.
Make sure that the build process runs entirely locally and never fetches anything from outside.
Simplify everything, use only tools and languages that are well understood and supported.
The real problems are not strictly technical but social: how do you prevent loss of the code, the tools, the specification, how do you maintain the expertise needed to maintain it. How do you ensure that all those things that are obvious to you now are written down in all their gory detail so that your great grandchildren will not apply their new and different preconceived ideas to the system?
Document all this on paper as well as electronic storage, make sure that version ids are recorded on every page as well as being available to the user of the machine or program.
In the industry in which I worked for the last thirty years of my career it was not uncommon to have things come back for repair after fifty years use and to be able to consult the original drawings and bill of materials so that exact replacement parts could be made.