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."

Both defection and betrayal are more emotionally-loaded terms than is brain drain. Note, the referenced wikipedia article uses a still more passionless phrase, "human capital flight". It also mentions converse term brain gain and related terms brain circulation and brain waste.

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.