A coworker at one of my former employers put it thus: People who can do engineering become engineers. People who get into engineering but can't do it become Six Sigma consultants.

I haven't seen companies actually promote incompetents into management. Usually they're just given their walking papers. The more charismatic or well-connected ones are given some sort of lateral career track, like from actual engineering into sales engineering or something. I have never been charismatic nor well-connected enough to pull this off.