> They probably do have exits though, in case another elevator breaks down.

Floor space is generally too precious for that. In a typical multi-bank elevator setup, lower floors get their toilets or other utility spaces placed between two banks of elevators that don't stop at that floor; the corresponding point in the upper floors is the elevator lobby for an upper-floor elevator bank.

There's a widely reported story that a group of people stuck in an elevator in the WTC on 9/11/2001 used a window washer's squeegee to cut their way through multiple layers of drywall to escape into a 50th floor bathroom.