What the flight path shows is consistent with the O2 bottle being shut off leaving enough O2 to initiate descent, then the pilots go incapacitated.
The high altitude chamber films show pilots recovering to a walking, talking state quickly after getting the mask back on - - after maybe half a minute without.
After several minutes of high altitude hypoxia, you will not suddenly regain your piloting ability when below 14,000.
Case in point: An Argentinian glider pilot woke up in a field with the glider missing the wing outer panels after being sucked up into a CB. He had no memory between the time he pulled the dive brakes and when he woke back up.
My O2 system will run down sharpish if I forget to open the bottle, but it will show up on the flowmeter before I get very high.
Before the quick donning masks, one pilot was required to be wearing a mask and breathing with it above 30,000 or so . A closed bottle would be quickly apparent and an RTB would be initiated if it could not be turned on in the air.
Today crews are betting their lives and those of the pax that mx has opened the bottle if there's a depressurisation.
Helios lost that bet and we may be looking at a repeat.