In the broadest sense, it is important for a company to have enough bench strength and cross training so that the departure of one key person will not send the company into a tailspin. However, to use this principle as a verbal weapon to intimidate and threaten employees is total mismanagement.
Your manager is operating under the misconception that threats are motivational. In the short term, his approach may indeed have a motivational impact as people strive to avoid being terminated. However, over the longer term, your manager's approach undercuts performance, commitment, and loyalty. If anything, his approach is simply going to motivate employees to leave.
The next time he utters his trademark phrase, you can tell him that you and your associates understand what he is saying, and you really do not need a regular reminder. However, based on the level of managerial competence that he is currently displaying, he might not take this the right way. You will probably be better served by ignoring his comments and regarding them as little more than background noise, which is the most common domain for overly repeated phrases. If your manager's comments are typical of his overall managerial behavior, perhaps his words apply most accurately to himself.
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