I mean, it doesn't take much for a class of errors that has existed for 55 years to cause a billion dollars in damage. Just in US labor costs, that's less than 5 hours per developer year. And I probably spend about 10 hours a year on average dealing with NPEs.
Convenient hand-wave to distract from the question of wether or not it has been a mistake at all: "look, reverse-extrapolated to the individual man-year it's such a small number! Who cares about the sign!"
That hour you spent chasing an NPE? Yeah, it happened. (really not that often though) The time saved by having a single, universally accepted way of implementing optionality, even it's inconveniently always on? That's invisible because we did not live through the alternative.
