On the treatment side the stakes are physical rather than informational. Hydrogen accumulates, and if the ventilation fans stop and nobody knows they have stopped, the building becomes an explosion risk.
A SCADA network carrying that kind of signal is not a monitoring convenience — it is what stands between a sensor reading and somebody getting hurt, which is why the redundancy went in first.
Outcome
Control that survived the loss of either site — put in before anything else was.