ezbzbfcg2 wrote:
And "left there," did they literally leave? Where did they go? What did they move on to? Or did they mentally check out?
You may already have the answer from researching the Peter Principle, but effectively, they are 'left' at that level in the sense of abandonment - they do not have the skills or experience themselves to progress beyond that level, and no-one is interested in promoting them (or at least not for the same reasons as the Peter Principle, where a promotion is based on competence at the current job they are in).
It is possible they may still end up promoted further in future, but it will generally be for different reasons - people above them may quit or be fired and promotions may be based on seniority, or there may be a reorganization affecting multiple people, or someone may arrange for their promotion via political maneuvering for reasons of their own. The commonality of these types of promotion are that they are not based on the person's individual merits or effort.