> At a large enough company you continuously lose people for natural reasons, like retirement, moving for a spouse, moving to be closer to family. Past a tipping point you're just always hiring.

As an analogy it's kinda like hard drives: when one of my hard drives dies, it's An Event which I need to deal with specifically; I need to order a replacement for that specific drive. At Google's server farms, hard drives dying is just a statistic, you continuously buy hard drives just because you know existing drives are gonna die at some semi-predictable rate.