During the Civil War, Abraham Lincoln was given a list of losses in a recent battle: 12 generals and 5,000 horses. Lincoln said, "That's too bad about the horses." His point was, show me 12 colonels and I can make 12 generals with the stroke of a pen. Making horses, however, takes years.
Similarly, in the US military if 1,000 of E-5s (Sergeants) are leaving he military, show me 1,000 E-4s and I can fill the shortfall. The question is the quality of those replacements: experience level, proficiency in job skills, judgment, maturitty, etc. The military can wallpaper over a hole in the sheetrock to make it look like everything is okay. The US may not realize it had a drop in quality until we get into a real shooting war, and then the consequences are serious. Politicians tinker with this stuff at the peril of the nation.