Or a more morbid analogy: military casualties in a war.

It's not really an analogy, it's just facts of life. In a large enough company or over a long enough time period at a job or in your career, realistically people actually will eventually die and cause vacancies. I've had two coworkers die, one because of illness, the other because of an auto accident. I stopped using the "bus factor of one" phrase as a result even and use "redundancy or succession planning" since it's not outside the realm of possibility that someone might get hit by a vehicle.

Sure, but in one many more people leave the workforce by being killed and maimed than in the other.

"Lottery factor" works, though it's more of a mouthful

That, and nobody knows what you're talking about.

Just the first time

More morbidly, military casualties in what the military insists isn't war.

Even in peacetime, training accidents kill an expected number of military personnel each year.

I'm not talking about training accidents.