I agree with the creeping hypoxia theory mentioned above, this would be compatible with the facts we have seen, e.g. the perfect straight line flying (unchanged heading) with descending without any radio communication. Both incapacitated and unable to inhale the necessary O2 before it was too late to recover from hypoxia and from descending into terrain. They probably only managed, already partially incapacitated but clearly unaware or unable to use masks, to enter the descending phase and then they could not complete the rest of the other instrumental changes meaning their plane continued descending without pilots until impact.
