Fire retardant is an emergency measure, one that would rightly be expected to see exceptionally low usage overall. But over time, more people and property have gotten closer to the forest; forest fires affect more people for many reasons.
So fire retardant use is not so rare.
The Therac-20 was a fine piece of electro-mechanical-nuclear technology, but the Therac-25 moved the control scheme out of its original context, and took away some of the physical interlocks. The Therac-25 is not remembered fondly.
Context changes over time, and assumptions need to be re-examined.