Boss refuses to let employee do their job while she's on leave, company hires someone externally who has no idea what to do and boss ends up with a mountain of unfinished work: 'You are not capable of doing my job'

It's nice to think that you're irreplaceable. It's that sense of security that lets you sleep at night, not worried that you'll lose your entire livelihood should your employer choose to replace you overnight. That confidence and knowledge can get you a long way and actually help you to be successful in your role. 

That said, change is inevitable, and it's almost certain that you'll eventually move on from your role or even need enough time off that someone needs to be seconded into it. Gatekeeping your job at this point isn't going to help anyone, including you, and it isn't a great idea unless you have a bone to pick with your employer. Sharing your knowledge and fostering collaboration in these circumstances can actually strengthen your position at your job and make you less likely to be replaced.

When this boss took three months off work, she swore that no one else would be able to do her job—she insisted upon it. So, despite having 28 years of relevant experience, this employee, her subordinate, was passed over for secondment to her role. So, the company hired an external temp to cover while the boss was off who had no idea how or what to do, meaning that the boss had a mountain of work to sift through when she returned… Handling a difficult situation like this with humor and patience can really help put people in their place. Maintaining self-worth in the name of subtle defiance.