I've watched a colleague go from a good engineer, to a great engineering manager, to an awful project manager. I see him hating what his career has become and I feel terrible for him.
For sure. It's absolutely NOT the expectation, but there definitely exist people whose talent is in people-managing with little to no domain expertise.
IMO, whether one of these individuals can be used is the distinction of whether a role is line supervisor or management. Line supervisors need the subject knowledge to teach fresh hires and know how to identify and fix problems caused by slackers. Management would rely on the senior employees for mentorship and keep their ear to the rumor mill to identify suspect underperformers.
