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he maintains the computer systems on a whole bunch of computer controlled manufacturing equipment, stuff made to last 50 to 75 years and they hang off the network as well.

In short, in a situation like that you do what the banks have done since they started using computerised banking systems back in the 1960s, you hire people with the skills to look after them and fix them in-house or go with an independent support service. If the system doesn't warrant that sort of expense, then maybe you don't need to worry about it. It all comes down to risk assessment and management.