The person on the other side gets the benefit of that. The airline still got $39 out of it. A more interesting scenario is what happens if both of those people

The person on the other side gets the benefit of that. The airline still got $39 out of it. A more interesting scenario is what happens if both of those people paid: can the airline sell the same empty seat twice? I'm assuming it's fine because they're not selling an empty seat, they're selling seating on a seat that has an unoccupied adjacent seat.