It is a common complaint in pyramid-shaped hierarchies that the "best" people leave. This is because they have to leave. As they climb the ladder, the rungs get smaller so fewer can fit. Each year, many of the people who were the best the year before (and got to the next rung on the ladder) have to leave. So people in organizations like this naturally come to think that the "best" people leave -- because eventually they all do. Of course, it is possible to design a hierarchy so dysfunctional that the best people leave and the worst people stay, and I don't know enough about the armed services to assess whether that's the case here, but whenever I read about the best leaving, I wonder how much of that's an intended feature or an unintended bug.