The source code isn't hiding in a repo somewhere for security reasons — it's spread around on various pieces of paper and computers over the last 50 years. There isn't a single source of truth. Adds a whole other level of wizardry to keeping the thing running.
How has nobody at the top ordered a digitization and consolidation of all known code and supporting documentation during that time frame.
Having a ton of people run around the office for a couple months to collate a bunch of documents so you can better pass info on to a new generation of workers when the satellite might not even be usable anymore isn't very efficient. Might as well just pay an extra 50% or whatever to the 5 dudes who know what's going on until the thing is inop. Even if it died today, the mission still would've been a massive success.