I don’t think there is that implicit assumption. My understanding of the principle is that a person would never get “promoted again”, because their incompetence at their level prevents it.
Essentially: if you’re good at a job, you get promoted out of it. If you’re not good at a job, you don’t get promoted out of it. So the end state for everyone is that they remain in jobs they are not good at.
Essentially: if you’re good at a job, you get promoted out of it. If you’re not good at a job, you don’t get promoted out of it. So the end state for everyone is that they remain in jobs they are not good at.