> Except pilots have a seperate oxygen supply for just such an eventuallity and besides, none of those would have caused the plane to do an almost 180 degree course reversal in stages over a period of a couple of hours
Please, read the link about the Helios flight - you'd be amazed at how badly people function with low-level hypoxia. I've been there, and it is extremely strange - despite acknowledging the symptoms, my reaction was a shrug, "oh well," and carry on with what I was doing - no remedial action.
Oxygen deprivation isn't an on/off switch. Let's say, for example, that cabin pressurisation failed for whatever reason, and the pilots were using their oxygen, but didn't realise they were suffering hypoxia. Maybe the bottles were low, maybe there was a leak, maybe they just weren't using it correctly if at all. Or maybe there was a slow enough depressurisation that they just didn't react correctly, and took a long period to decline to unconsciousness. Re-routing would be a perfectly believable action. Who knows what was going through their heads.
Maybe these scenarios seem unlikely, but something very unlikely has happened - so the explanation is also bound to be pretty unlikely.
I still agree that a deliberate act seems likely, perhaps more so than the scenario above, but unfounded statements of one explanation being the "only" logical one are simply wrong. As above, it is a tragedy the plane won't be found, primarily for the families of those involved but also for the lack of ability to learn from the accident and, potentially, to prevent a repeat.