Regardless of the size of your organization there is a problem that all organizations share: The knowledge of how things work resides largely in the heads and experience of the people who designed or have been doing the work. When they leave, the organization loses 'know how' that may not be regained easily.
Even if key personnel are temporarily absent, no one else knows what they know, or how to find out what they need to know, to get the job done.
The real cost of the unavailability or loss of key and core knowledge is decreased product and service quality and loss of reputation.
Financially: If this is not rectified quickly, it leads rapidly to brand damage, reduced market share and shareholder value and in many cases the financial instruments of the business can not support a reduced share value.
When key knowledge is perceived by the market to reside in one or two key individuals, those individuals can never leave without potentially destroying their legacy.