This story is false according to Musk, which was widely reported when this first hit back around August

http://mashable.com/2017/08/11/elon-musk-fired-assistant-ane...

According to Musk, "Mary Beth was an amazing assistant for over 10 yrs, but as company complexity grew, the role required several specialists vs one generalist." She was given, Musk claims, 52 weeks of salary and stock for her contribution & left to join a small firm, once again as a generalist.

The story is false, yada yada. The lesson isn't the one they're pushing. The real lesson for not being replaced and increasing remuneration is:

"Know where the bodies are buried"

Musk: "[Author of story] never actually ran this story by me or my assistant. It is total nonsense." - http://www.businessinsider.com/elon-musk-responds-assistant-...