I don’t know if the military is copying civilian patterns or vice versa. But in the military the Line Manager is a noncommissioned officer, and if they want to

I don’t know if the military is copying civilian patterns or vice versa. But in the military the Line Manager is a noncommissioned officer, and if they want to stay that role forever that’s fine. As long as you can keep up with your unit. But commissioned officers are more “up or out”. If you don’t have a trajectory that gets you to stop being a lieutenant someday, then odds are very good you’ll be encouraged to leave. Not everyone has to end up a two star general, but you aren’t meant to clog up the ranks for people younger than you who have the potential to be one. The Gervais Principle suggests that you keep a few of these around in case upper management tries to mark their territory by asking for consequences (usually for their own decisions) so you have someone to throw under the bus. I don’t think that’s wrong, but conservation is often very different from thriving. It’s an artificial state that is still precarious and lacking agency for the subject.