The "bus factor" is a way of saying "how many devs could we lose on this project and still have a working knowledge of it?"
How many devs could win the lottery and retire to a desert island while we still retain working knowledge of it.
Whereas even someone who won the lottery will take a $100k/hr consulting gig, or might take pity on his ex-coworkers and explain his choices in a 1 hour phonecall while sipping margaritas on the beach.
But it’s a reminder that they wouldn’t help if they had financial security.