In a collapsing dungeon, the rule about removing a room with any players still in it seems odd.
You may not remove a room with any players still in it unless there are no rooms without players in them remaining on the lowest floor of the dungeon, or unless the only room without players in it is the room with the stairs up.
If you look at the example on page 29, it seems that the removal of room 10B is forbidden due to the empty non-stair rooms on the floor: The Chasm of Unnecessary Cliffs, Dungeon Corridor, and A Dark Room.

For that matter, if a floor has more than one set of stairs leading down to it, it seems that it would be difficult to remove a room once that floor is reduced to the rooms below and between the stairs.