What ends up happening is that they'll bolt on some network connectivity for convenience or to take on some new process and not set it up appropriately, or not understand what it means to expose something to the LAN or directly to the internet.
I helped a friend at a municipal utility with something like this when they wanted to provide telemetry to a city-wide operations center. They had a dedicated LAN/WAN for the SCADA stuff, and the only interface was in this case a web browser running over XWindows that had a dashboard and access to some reports. I think they later replaced it with a Windows RDS box with a similar configuration.
Because of the isolation, and professional IT who understood how to isolate the environment, it was advisable to to not be tinkering with updates, as the consequence of failure is risk to health & safety.
(I'm not advocating for HVAC/SCADA systems to be running, say, Windows XP Embedded with no updates and default passwords, world-facing, just observing that the preconditions changed.)