Not really. "Key developer gets a new job" means they are still available for you to ask questions to, get help locating passwords/keys from, and so forth. In the "key developer gets hit by bus" scenario, you don't have that luxury.
The idea isn't so much about being hit by a bus as "what would we do if a key contributor were to vanish off the face of the Earth with no notice or warning?" If the specific idea of the Death Bus seems too grim, feel free to replace it with another deus ex machina that would have the same effect (alien abduction, Christian rapture, surprise release of a new season of Firefly, etc.).
The idea isn't so much about being hit by a bus as "what would we do if a key contributor were to vanish off the face of the Earth with no notice or warning?" If the specific idea of the Death Bus seems too grim, feel free to replace it with another deus ex machina that would have the same effect (alien abduction, Christian rapture, surprise release of a new season of Firefly, etc.).