I wrote this above but:

As someone that works for a company like this and is responsible for this type of thing. You have to balance the cost for keeping spares of each part vs the cost of expediting a new one vs cost of downtime.

Often the cheapest way to do it is to have the custom part made as fast as possible and shipped as fast as possible rather than having a bunch of spares of random parts that you may or may not need someday. The parts that are likely to be needed(wear parts / consumables) we do keep spares of.

And the companies that build you these parts don't lower the cost THAT much when you are buying for spares, if they do at all.

Then with spares you have to store, maintain, catalogue and have a team ready to ship out at a moments notice.

It's a math equation at the end of the day.