I gave the danger of the bus number one lecture to a bunch of managers once, and had one pushback pretty hard by saying that what he was working on was so compl

I gave the danger of the bus number one lecture to a bunch of managers once, and had one pushback pretty hard by saying that what he was working on was so complex that no one else could possibly learn it in time to be useful for the next release (every release...). They bought me lunch a few months later to apologize after having the unique person die and leave them with no backup. -- I have a relative who just retired and was replaced by five people who started a year ago. It is extraordinarily difficult to replace decades of knowledge and wisdom.