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Every software company I've worked at that is more than 5 years old had major features that nobody understood anymore, even features that were core to the product.

Dont forget the critical software that keeps the company going that someone dealt with long ago, and was left to rot when they left, only for someone to discover it and have to go on an archeology dig to find info and improve upon it.

The gear cutting machines almost never had mechanical failures. As long as they keep them lubricated, and occasionally muck out the sumps, the machines should still be going in the year 2100.

The other thing about gear cutting is that hobs only cut one size/profile of tooth. Some of the cutting tools I was using dated from before WW1, for odd sizes that didn't get used much.

> for odd sizes that didn't get used much

Well, perhaps the long term solution is to rework it and start to use more common sizes. Standardization is a good thing in the long run.

This happens in tech as well. It's called COBOL and it refuses to go away, despite lots of people's best efforts.