Your understanding of stuxnet is flawed, Iran was attacked by the Us Gov in a very very specific spearfish attack with years of preparation to get Stux into the enrichment facilities - nothing to do with lifts connected to the network.
Also the facility was air-gapped, so it wasn't connected to ANY outside network. They had to use other means to get Stux on those computers and then used something like 7 zero days to move from windows into Siemens computers to inflict damage.
Stux got out potentially because someone brought their laptop to work, the malware got into said laptop and moved outside the airgap from a different network.
"Stux got out potentially because someone brought their laptop to work, the malware got into said laptop and moved outside the airgap from a different network."
The lesson here is that even in an air-gapped system the infrastructure should be as proprietary as is possible. If, by design, domestic Windows PCs or USB thumb drives could not interface with any part of the air-gapped system because (a) both hardwares were incompatible at say OSI levels 1, 2 & 3; and (b) software was in every aspect incompatible with respect to their APIs then it wouldn't really matter if by some surreptitious means these commonly-used products entered the plant. Essentially, it would be almost impossible† to get the Trojan onto the plant's hardware.
That said, that requires a lot of extra work. By excluding subsystems and components that are readily available in the external/commercial world means a considerable amount of extra design overhead which would both slow down a project's completion and substantially increase its cost.
What I'm saying is obvious, and no doubt noted by those who've similar intentions to the Iranians. I'd also suggest that the use of individual controllers etc. such as the Siemens ones used by Iran either wouldn't be used or they'd need to be modified from standard both in hardware and with the firmware (hardware mods would further bootstrap protection if an infiltrator knew the firmware had been altered and found a means of restoring the default factory version).
Unfortunately, what Stuxnet has done is to provide an excellent blueprint of how to make enrichment (or any other such) plants (chemical, biological, etc.) essentially impenetrable.
† Of course, that doesn't stop or preclude an insider/spy bypassing such protections. Building in tamper resistance and detection to counter this threat would also add another layer of cost and increase the time needed to get the plant up and running. That of itself could act as a deterrent, but I'd add that in war that doesn't account for much, take Bletchley and Manhattan where money was no object.
I once engineered a highly secure system that used (shielded) audio cables and amodem as the sole pathway to bridge the airgap. Obscure enough for ya?
Transmitted data was hashed on either side, and manually compared. Except for very rare binary updates, the data in/out mostly consisted of text chunks that were small enough to sanity-check by hand inside the gapped environment.
Stux also taught other government actors what's possible with a few zero days strung together, effectively starting the cyberwasr we've been in for years.
