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.

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