(by plum brook, do you mean NASA? If so, I've met some smart people from there and other NASA posts, but also met some of the worst employees that epitomize some of what the discussion is against - very toxic, against any new ideas, refusing to retire, shirking duties etc. at one point one org had an average age of 59+, that's very indicative of a refusal to move on).
>it is also not terrible at hiring very competent/smart people through their commission system
I suspect you would get a very different perspective if you talked to enlisted servicemembers (or subordinate officers). I would argue the commissioning system is better than the previous aristocratic commissioning system, but still relatively poor at mating skills to positions.
I think churn is a couple of problems for a couple of different potential reasons:
1) constantly moving positions tends to leave the more complicated problems unsolved. For one, it's difficult to truly understand the dynamics of a complicated system in a short period. Secondly, if someone is concerned with promotion, attacking small problems tends to get you a win during your tenure, while it's unlikely you'll make much headway on a really difficult or complicated problem. Even worse is the commander who has all kinds of great ideas they want implemented even before they really understand the problem (ie the 'good idea fairy' dilemma)
2) military churn can bias toward giving people responsibility beyond their capability, simply because they need someone to fill that role. This is especially with younger organizations (and the military definitely biases young). Meaning you tend to people with a lot of power/responsibility before their frontal lobe is even fully developed.
Now I do think the military does a pretty good job at accountability, which can mitigate some of those factors. But if that's the case, we should be trying to optimize for "accountability" and not "churn".
The better military organizations seem to have "churn" in the uniformed services in charge but a steady cadre of professional civilian staff supporting them.