Often there are people who can literally do the job of five people (jack of all trades sorts). Then there are also people who have such specialized knowledge or deep insight into a particular domain that they may simply be irreplaceable. Documenting “what these people do” is often the first step to driving them to work elsewhere. If you are worried about retaining these people, try paying them what they are worth, or better, granting them authority equivalent to their responsibility (ha! I’ve yet to see that in many businesses).

What if they left? What if you never had them in the first place? What would happen to Apple if Steve Jobs died?

The answer is always the same: you’ll just be another boring business doing stuff that anyone could do because no one working for you is too special.