I haven't done any work for the military but i can say that all the legacy systems I've worked on were because the specific software they need was written only for Windows 98 and the developer or company that created it is long gone. Keeping it going is a chore but switching to literally anything else is out of the question.
I could see for military applications that having the known quantity of a working piece of software that isn't changing anymore and can be swapped as an entire unit is an advantage, especially if it doesn't touch the internet in any capacity. But eventually you run out of people who know what to do if any changes need to be made.
Octoprint is what I use. Slicing is probably the thing it woukd be least good at but all the rest is good. And theres an api to write plugins for if youre into that sort of thing.