People tend to get "promoted" when they outgrow their current role and need more challenges.
But eventually all will reach a point where they stop growing or grow content or complacent and the promotions will also come to a halt.
It doesn't necessary mean they are "incompetent" though. Just stopped growing bigger than their current role.
https://news.ycombinator.com/item?id=42559052
4-5 years before founding TSMC, he was forced resign from Texas Instruments after 20 years there and then got fired or pushed out from 2 other companies in a few short years.
He finally and unexpectedly hit the jackpot after this, but it could have ended a lot worse.
Growth rarely seems to be something linear or predictable. There can be lots of ups and downs, of things that seem like contractions and setbacks, instead of growth.