Not crazy at all! I have been that bus-factor-1 employee. I raised my concerns to the CEO several times, and it always looked like this: "I am a bus-factor-1 e

Not crazy at all! I have been that bus-factor-1 employee. I raised my concerns to the CEO several times, and it always looked like this: "I am a bus-factor-1 employee, this is bad - please teach somebody else in the company - I tried and failed, nobody has the prerequisite knowledge, nobody understands me when I try to teach". This looks very much like the situation that you are describing. I'd say that "bus factor 1" is not necessarily the root cause of the lost knowledge - it might be a consequence of other hiring mistakes. I had also seen this in a different company (but not in the context of a person leaving) when we hired a single machine-learning specialist. I watched his struggles to communicate with everyone else who didn't have the background. In a different company, I was on the other side of the stick - an employee who did not have the prerequisite knowledge to be the target of knowledge transfer. That's also really frustrating - due to not meeting the (unreasonable) expectations and "learning too slowly," I had to go to a psychiatrist and eventually left.