Well I didn't add this but I would stipulate that the secure side would have almost no permanent memory at all if possible. I mean, we've been controlling boats without electronics for millenia so if you make it a priority to have no permanent memory, it should be achievable.
It's doable. The biggest issue is that all these engineers are gonna cost $$$$ to design these systems and you will need to do a lot of QA, which also costs $$$$.
It could be doable to transition back to pneumatic PID blocks by some royal decree but it’s definitely not going to be any real government’s solution. PLC’s are here to stay for all complex machines, and these ships are relatively complex.
More interesting to talk about options that could realistically happen, and discuss pros/cons of various government/industry solutions that are actually likely to occur.
I wish I could find a cutaway of a pneumatic PID block though. They’re quite amazing technology that implemented true P-I-D “calculation” logic in a purely physical form by using pressure of air at two inputs (setpoint, current value) to control one output penumatic pressure which in turn would control some valve a distance away. Really amazing engineering we had before electronic control! The air lines had a bad tendency to get clogged up though.
It's doable. The biggest issue is that all these engineers are gonna cost $$$$ to design these systems and you will need to do a lot of QA, which also costs $$$$.