Perhaps we should pass a law that no utilities / infrastructure should be attached to the internet. Private networks are fine for this purpose.
Some critical infra is air gapped though. Other systems implement SIS systems in parallel with general process systems to mitigate catastrophic failure further.
There are devices called data diodes that provide unidirectional network topology, but not all time series data interfaces can work with them.
All in all, I agree that total air gap is obviously the best way to mitigate network attack vectors, but sometimes not practical. No controlling device should be at level 3 or 4 though (business or enterprise level).