Comment by h0l0cube

I sort-of kind-of agree with this. It is certainly why most of these systems aren't worth upgrading. But reasons why it might be desirable to wear this cost is if the original system is already showing signs of bugs, which may be true if it's being operated in new conditions (e.g., interop with new systems, increasing workload), or there are safety features that could be added but not without refactoring. But, yes, if it ain't broke, surely there's no need to fix it

Edit: Also worth pointing out that formal verification was likely not a component of building the original software, and it likely has bugs it's kept since inception. Updating the API to a language more conducive to formal verification might be an option that improves safety over the legacy system

Edit: Apparently they did replace it all eventually

https://news.ycombinator.com/item?id=29158255