When the possibly of losing $200k an hour with a line down even exists, "screw it, I'll have a few spares made" seems like it would be a less silly idea the plant manager might want to consider.
I remember fighting to get some UPSes on important machines once upon a time. I don't remember going even one single summer without having a power outage. In fact, there were normally several per year.
When you're 'only' earning an average 20K$/h (10% profit is an ok number I would say, but it may be far less if it's a common finished good), then it may have sens not to pursue buying couples of every single spare part you should have. The full price for always having in stock working spare parts for 100% of a 3M$ production line could be well in the tens of hundreds per month. At the end you're cutting your profit and that's the last thing you want to report to the board, as a plant manager.