Yeah I would say that is a double edged sword. A lot of times it is the employer who forces that employee to become indispensable by continuously giving them more and more tasks instead of hiring another person. I wouldn’t ever say it is the employee’s fault for doing what is asked of them. But I agree it can be on the employee to help train others if they are truly that great at their job. But usually in that case the person who is indispensable should be promoted.