UNCTUOUS's theory is interesting but I just wonder had the cockpit been severely damaged by an O2 fire, just how stable the aircraft and its control systems would have been.
Certainly any hole in the side of the aircraft would not have initially or later disabled its ability to stay aloft but the control surfaces are fly by wire so the questions have to be asked, in the light of the destruction to the Egyptair example, could the controls remain in a detente position exactly as they were at the time the cockpit end of the system burnt through? How would they cope with turbulence changing any of the axes of the aircraft? If there was time to handle the aircraft to turn it round, why did not the crew call Mayday a la Swissair 111?
How could anyone regain the cockpit and perform any useful function if the damage was as extensive as the Egyptair example and for that matter to the Swissair 111?
Had Swissair 111 suffered its fate an hour or two later we would likely still be looking for it given the hypothesis set out above as it would have been in mid Atlantic with, on the night, not the best HF comms, though the VHF guard frequency might have been used.
My money always has been and still is on a hypoxia situation but, like everyone else, I'm currently lost for a cause that ticks all the boxes.