Jagman1: Yes, your concerns have been mentioned on another forum. However, apart from the following answer, QF 747-400s are fitted with extra oxy cylinders for terrain clearance.
Every flight plan calculates all contingency range configurations including engine/critical systems failure and depressurisation. The SYD-EZE route will be planned in accordance with CAR's, CAO's, AIP's etc and the route will be such that the aircraft will have sufficient range to divert to a suitable airport at all stages of the flight. Some posters on this thread have commented that it was lucky that this event did not happen on the previous sector wrt high terrain adjacent to the Tibetan plateau. Every segment of that route has been analysed and "Escape Procedures" have been published (& which both pilots at the control seats have in front of them) to take into account Single Engine Failure, Two Engine Failure and Depressurisation cases. By law, RPT operators are required to comply with these requirements and at no stage is an aircraft going to caught in a no option situation.