Likewise the idea that there are no "network ports", hence no way for modern systems to get access. This probably also that the whole system has no "network" security, bypass the security console and you have direct access to the entire launch system, because it never occurred to the creators that you could spoof the entire console. (The equivalent of the old Windows password you could bypass by hitting "cancel" on the "Try again: Yes/No/Cancel".) So if someone can smuggle something small past the, probably impressive secured, airgap, there is no second line of defence. Unplug the existing terminal, plug in a tiny portable bit of modern, hard-hacked kit, pwn the whole system.
You might argue that the techniques necessary are not routine hacker knowledge. But Stuxnet was not created by a script-kiddy. They had a deep understanding of the system they were trying to sabotage. This is a nuclear missile silo, you can reasonably assume a motivated attacker.
The scenario you describe means that they would have had to bypass several layers of physical security and also remove/compromise the two people at the missile command consoles, which are probably also armed.
The chances of someone successfully pulling off a plan like this is so insignificant it would never happen, unless its in a movie.
Face it. The system is about as secure as it possibly could be.