To be fair, there was a bit more going on there than straight up assumption of risk; the particular situation was that an airline crashed, and was likely liable to its' passengers -- but ONE of the passengers, had she reached her destination, would have been joining a mountain climbing expedition which was killed basically the day after she would have safely arrived, in an avalanche. The airline was arguing that they didn't owe that particular individual full recovery, because she would have died anyways the very next day -- so they were really only liable for one day's worth or her life, or so.
The treasure chest argument goes that, if hypothetically instead of an avalanche killing everyone, the group had found a treasure chest full of money instead, could this girl or her estate sue the airline for loss of what would have been that girl's future share of the treasure chest which she missed out on? The argument is intended to show that no recovery is the predictable, parallel argument: because we wouldn't allow an individual to sue the airline for missed good fortune, we shouldn't allow the airline to benefit from missed bad fortune (or something very close to that).
Eesh. Torts at Chicago really IS a gem.