They don't disappear. I use 80 sub out, 85 sub in. The players do take longer to sub back in towards the end of the game because they're tired. But they still come back once they've reach the 85 rested mark. I like getting game experience for my back ups in CFM. So I don't mind if they play an entire series towards the end of the game. But my starters always rotate back in once they reach the 85 rested mark.