I want the loss of anyone in the company to hurt us, but not fatally, including the likes of me. Every job performance review I give my direct-reporting managers includes the question, ‘Who’s your replacement? If you don’t have one now, I can’t consider you for bigger things. If you don’t have one the next time I ask, you may no longer be a direct report.
Who’s Your Replacement?
Joe Lonsdale

[1] Bloomberg, Michael. “Bloomberg by Bloomberg.” John Wiley & Sons, Inc. 2001.