As a new lieutenant, I witnessed the early retirements and SERBs of the early 90's. Typically these are geared to weed out the middle ranks. The 15-20 year careerists they target will be majors and lieutenant colonels, as well as as some captains with enlisted time. This isn't so much about emptying the current upper ranks as it is about grooming the next generation.
I'm not military, so I'm talking out of my butt, here....
But it strikes me that in an organization like the Marines, a little turnover might not be a bad thing....
....Provided that it's not "politically motivated" and done properly.
I've no doubt about the latter. Marines always get things done right. The former, though, I'm not so sure about.
I work with a Marine who has fairly significant issues with his back....those 100-pound backpacks probably didn’t help. He’d likely be the first to take up an offer like this.