It's arguable. No one is so valuable that they could never be let go, laid off, or fired, regardless of circumstance. That much is true, sure. But I have certainly seen projects veer off course, wither and die, or else crash and burn due to a coworker's involuntary departure. They were irreplaceable. Not that it saved their skin. Projects with a bus factor of roughly 1 happen from time to time, so the person who happens to equal that factor is de facto irreplaceable. The organization will move on as if everything's fine, but the loss of certain individuals may be far more expensive than anyone in charge is willing to acknowledge.