Airline requesting passengers to voluntarily move to later flight?

[Zac S]

Due to the fog on Saturday, our Virgin flight was delayed by 30 mins, however due to the international diversions, 12 crew were required to travel down to Sydney, however originally they had problems getting enough people to move to a later flight, even with the offer of a refund for the cost of the flight. Eventually it reached the stage where they said the flight wouldn't take off until someone else gave up their seat, luckily someone gave up their seat.

So what exactly can the airline do in this situation if no one gives up their seat?