I don't think their wealth is relevant to the terrible fate facing them by the end of this day. Dying slowly, entombed in total darkness is not a way anyone would choose to shuffle off this mortal coil. I'm sure the great-great granddaughter of a chap who drowned on the Titanic with his wife wouldn't appreciate the irony.
The rescuers are trying to instil a sliver of hope by saying the oxygen may last longer if those trapped on board can conserve their supply but even then, they still need to locate the precise position, winch down the 20,000' long cable, weld it to the sub', then winch it back up.
Mission: Impossible.