Step away from the military side for just a second. In the finance world where systems are just as critical, that have to be near 100% stable, they purposefully use old very stable, Operating systems and software. The idea is that after 15+ years M$ has figured out the vast majority of the bugs and released patches for them. Many Bank ATM machines are still loaded up with NT4 because it is near bullet proof after 15 years. Same goes for hardware. Every brand new spanking processor comes with a page full of errata. After many years software and OS houses come to work around and develop ways of avoiding all errata events.
Speaking of reliability IIRC there is a OpenVMS machine in Ireland Railways that has operated for nearly 20 years non stop. :D