Actually it's possible to implement such a program so that the airline only risks 1.5 empty seats. At seat selection, ask which seat the first purchaser would like. The next person buying a half-empty seat would only have the option to buy the seat on the same row.

As for Crabbing's questions, if the airline wants to put someone in the seat you have already paid for they should pay a bump fee. Seat poachers would be ejected.