Should the Army keep pushing in new soldiers while barring current soldiers from re-enlisting in their current MOS?

It's not as simple as "the Army needs XXX amount of people".

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 The Army needs a certain amount of people at each skill level per MOS. It needs X amount of Privates, X amount of SGT's, X amount of Generals, and so on, and those numbers are even further subdivided by MOS's, Special Duty assignments, specialty tracks, etc...

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You can't just stop assessing people into the military for the sake of the people already in. It's not that simple. There are countless moving pieces, every single one of which influences and interacts with all the others.

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Rest assured these things are not being done arbitrarily. Managing personnel on this scale is a MASSIVELY involved undertaking with more factors than you and I can discuss in ten years, much less a quick RP discussion.

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With stuff like this, a good rule of thumb is, if at any time you catch yourself thinking   "this is simple, all you have to do is xxx" it means you don't understand it. These things are amazingly complex. No one person is in charge of it for a reason.