[pembrook]

In nearly all mature, publicly traded firms, if the death of the CEO would cause a big disruption for the firm's future prospects, then it is not a well run business. The decisions required at 99% of these mature companies do not require Steve Jobs-esque foresight nor risk taking.

[o_class_star]

Precisely. These businesses exist to minimize risk, which includes personnel risk. Everyone should be replaceable, even upper management. (Which is to avoid the psychological costs of replaceability or of investing oneself into work one knows that plenty of other people could do-- that is not something companies concern themselves with.)