This is not in response to Brown specifically, just in general.
The Army has a finite number of MTOE slots for every MOS of every rank. Typically, the higher your rank, the fewer slots there are.
I know they went to a different retirement system since I retired back in 14, but I think for most of the senior personnel they'd still fall under the pension plan. Once you hit 20 years you receive 50% of your base pay as your retirement pension. For every year you serve after that you receive an additional 2.5%, which would be 75% after 30 years and so forth. Many people hold on for the money and don't want to retire.
What this would lead to is a backlog of soldiers with a promotable status not being able to move up in rank. The further down the line you go the bigger the backlog was. We had alot of sergeants and staff sergeants that were forced out for meeting their retention control point because a slot never opened up for them to be promoted into.