Upgrading control systems in a plant is both costly and risky. You incur significant downtime to replace expensive equipment that has a really long life time. And then shaking down the new systems is going to cause additional costs and likely also a temporary decrease in the quality of the plant's output. This is why the industry is basically stuck in a state where the most widespread standards originated in ancient, sometimes even analog times and have had tons of extensions tacked on in ways that preserved compatibility. So all of these devices operate based on pure trust. Bad input is rather attributed to failures than deliberate malicious actors inside the system. Nothing is authenticated. None of the field bus systems I am familiar with could be upgraded to incorporate that kind of distrust between components.
