Eh, my experience is that a lot of things in the military are sold as systems, and that includes the OS that goes along with it.
So fire, or demote, the idiots who did not plan for a known future event (EOL of XP) when purchasing these systems. That includes the Generals, Admirals, etc..
It's not that easy. If they were migrating the OS and OTC software, it wouldn't be so problematic.
But they run millions of lines of custom code, purpose written for specific devices.
Changing OS would also require rewriting (and testing) all those specially apps.