What is the best word for "brain drain" within a company?

Solution 1:

The typical word for the general phenomenon of employees leaving an organization is attrition.

This word in reality can have a more significant meaning in the sense of loss of intellectual assets, thus causing a reduction in the company's overall capabilities.

So, I think you can use attrition in its primary sense or its metaphor here.

Solution 2:

Brain Drain can be used in the situation you described, as it means:

a loss of trained professional personnel to another company, nation, etc., that offers greater opportunity.

Merriam Webster Online define it as:

the departure of educated or professional people from one country, economic sector, or field for another usually for better pay or living conditions

Solution 3:

I'd consider defection (an act or incidence of "abandoning or turning against; ceasing or changing one's loyalty") suitable in the context you describe, on the basis that when "knowledgeable people change jobs ... or leave the company ... without imparting that knowledge to others", they have acted disloyally. Also consider betrayal, as to betray has a sense meaning "To prove faithless or treacherous to, ... ; to be false to; to deceive; as, to betray a person or a cause."

The term brain drain is less emotionally-loaded than are defection and betrayal. The referenced wikipedia article uses a term with even less passion in it, human capital flight. It also mentions converse term brain gain and related terms brain circulation and brain waste.