We've got this silly tendency to glamorize the concept of being irreplaceable. "Yay!" you say, "I can be so cool that, even if I have a bad day, the company will lay every other engineer off before they get rid of me! I'm irreplaceable!" This is often about luck. Being the last guy to actually understand JCL at a shop that can't quite get rid of their last mainframe, for example. This isn't something you plan for.