This is obscenely true and somewhat underrated.

Sad as it is, the prime reason for the strength of DoD computer systems, both afloat and in the air, is that it's difficult enough to use and interface with them-- 'Hacking' them, and getting the ability to pivot and deny/destroy is extremely limited. Sailor Joe Schmuckatelli's Fire control system is on a separate network because there was no common network at the time the contract was let for competition. It's both the worst and most extreme version of security through obscurity. Now you're introducing more connections, availability, and reliance on a core infrastructure-- Which still won't meet basic corporate cybersecurity / patch control doctrine.