That is completely impractical.
People in userland need data from the SCADA network to keep the business running. They absolutely must have a way to get it. Saying "no" isn't an option.
Sure it is.
Watch this: "You're being paid to do a job. Being inconvient helps to safeguard the public utilities and prevents tampering from remote locations. If I find any systems that are connected to the public internet in any manner no matter how convoluted, I will fire the responsible individual(s) and their manager(s) on the spot."
See how easy that is?
Need data? Write it to a DVD and sneakernet it to whoever/whatever needs it.
Good advice. Try it with 30 plants covering a 1500sq mile area. While you were out all day updating your servers, an instrument tech forgot to clean his thumbdrive before plugging it in to an IEM to update the firmware. Since you didn't have regularly updating anti-virus, your whole network is now down and the company is losing millions of dollars an hour in lost production while you try to clean the 60 servers and 400 consoles on your SCADA network.
That's even more of a reason to not be connected to the net. The damage would be limited to the area one man could travel in a day, instead of everything, everywhere.
And you know what? I don't care if it's practical. Not all jobs get to be "convienient".