1) A new key developer would be hired to maintain the project, and things would be a little rough for a couple months but the company would survive.
2) Every decent person would care more about the key developers life than the "major project".
This so called "bus factor" has always bugged me. Documentation is good, but talk of untimely death by bus is silly.
The reference to untimely death by bus strike is an example of macabre geek humour; not a serious suggestion that the greatest threat to developers is posed by motorized mass transportation.
Most often developers leave projects -- and especially open source projects -- for far more banal reasons: They move to a different position and don't have the time or inclination to continue maintaining their old code.
Can someone else get access to all the company-owned code the developer was working on? Is someone insisting that the developer doesn't keep three months of work on a laptop? Who else can get access to the passwords and keys for company-owned servers and services maintained by the developer?