The Peter Principle is a joke taken seriously. Is it true?

The Peter principle states that “every employee tends to rise to his level of incompetence”. If someone is good at her job, she’ll be promoted into a job that demands different skills. If she’s good at the new job too, she’ll be promoted again, requiring yet another set of skills. One day, she will arrive at a job for which she is wholly unsuited, and there she will stick. Since when did a manager ever get sacked for anything?