JH:
"Under Major League Rule 40 (a), the pool of players eligible for the postseason consists of the 25 players on the active roster and any players on the disabled, bereavement, suspended or military lists before midnight ETon August 31."
Per OP, players whose seasons had ended were brought on at seasons end. This sounds very much like the playoffs in MLB. Those teams cannot trade, pickup, or add ringers. So, even the Pros have limits on end of season additions.
I know the rules, thanks. There are thousands of professional baseball players that aren't in the Major Leagues. I've seen scenarios similar to the OP happen several times. It's pretty tough to argue that roster changes based on performance don't happen in the pros when that's the most profitable arena for the sport, and the profit is driven solely on performance.
I'm not saying it's fair, but it's the way things work. Perform, or don't play.