That said, her irreplaceability is more due to stupid security setup (she's the only one that has all the necessary privileges), so I guess she could be replaced eventually after some hiccups (and at a big hit to productivity).
Day to day experience may make it seem like if John or Jane got hit by a bus, nothing can happen.
In reality businesses adapt and there are probably other people that can step in. Once they must, they will.
"The bus factor is a measurement of the risk resulting from information and capabilities not being shared among team members. It is also known as the lottery factor, truck factor, bus/truck number or lorry factor".