Why not tell the last five people to book that they are on standby?
Not a bad idea. In the future, if you purchase a ticket for a flight and you are not assigned a seat number, that very well may be because you are one of the "over-booked" passengers and you may be denied boarding if everyone that has been assigned a seat shows up for that particular flight. If you have a ticket without an assigned seat number, you should go to the airlines ticket counter ASAP upon your arrival at the airport and ask that you be given a seat number. In most cases, the gate agent will bump some other passenger out of his seat to oblige you. Like I wrote previously, if the plane seats 200 passengers and everyone of the 205 passengers booked show up, five people will not be getting on that particular plane.
This type of action understandably really upsets those passengers that find themselves without a seat. I have seen people try to bargain with a passenger that has an assigned seat, so that they may get to their destination without having to wait on the next plane. If you have ever watched the movie, "Home Alone" and Kevin's mother is trying to trade a passenger for their ticket by offering them money, jewelry, etc., that part is mostly true. I have seen this happen more than once.
I remember on one such occasion when we were really over-booked on a long flight from New York to LA and we had something like 15 passengers without a seat. The gate agents were telling the unassigned seat passengers that there just wasn't enough seats to be able to board everyone. Then, one of the passengers asked why the airline doesn't just pull a larger airplane out of the hangar, so that everyone could be accommodated. Sounds like a great idea, but I don't know of any airline that keeps spare planes at airports, so that they may be able to fulfill this type of need.
I have always suggested to people that have asked me how they could make certain that they get boarded, if they do not have a seat number on their confirmation to report to the airlines's ticket counter immediately after getting inside the terminal and request that they be assigned a seat number. Without a seat number on your reservation itinerary, you're kind of left out in the cold. Events like this cause a massive headache for the gate agents, but the airlines don't give a hoot. They train their agents to deal with it and normally, it is handled without a lot if fuss. Gate Agents make only about $12-14.00 per hour to have to deal with some of their irate passengers, so in turn, this will also cause a lot of turnover at different airlines.