Maintain

Let’s take aside all humanistic considerations and ask the simple mathematics question: In that military system, how to maintain a senior-to-junior ratio as one-to-one?

Very simple. There are limited number of ways to make such a system sustainable:

1. When those in the senior grades leave the system well before the age of retirement. (The serious arithmetic answer is if all seniors resign from the army at the age of around 48.)
2. When a substantial proportion of those in the junior grades leave the system without becoming seniors. (Once again, the accurate numeric solution is if half of the juniors resign from the army after 12 years in the system, i.e. just before they get a promotion.)
3. When you change the configuration of the system and ask all Colonels to do junior duty. (If you follow the calculation so far, it would be immediately obvious that the system would be stable if all Colonels continue to serve as a junior for 6 years.)

Sounds logical eh? The very point to note is these three are not solutions to the problem, but consequence of the system. It goes without saying that all three of them contribute to the stability of the staff-mix; all that matter is which one is more likely.

And, the sobering truth is, if those in senior grade have little intention to leave (as they have all the benefit), and neither do the juniors (because their prospect of promotion is secure), the inevitable happening is to ask those less senile seniors to do junior tasks.