Why fly all that way just to commit suicide?
Whilst logic might not be the strongest suit of air pirates, or rogue crew, or terrorists, neverthless it seems to defy all reason for a conscious crew or even a conscious hijacker to fly on for hours if all they're going to do at the end of it is crash the plane.
Surely the most logical explanation is that the plane has been flown to a specific destination .Clearly that destination isn't an airport - otherwise we'd all know about it by now and wouldn't be having this conversation.
But the destination could be a kind of "Dawson's Field" landing strip somewhere - similar to that used by terrorists in the 1970 triple hijacking. Dawson's Field was , I think, in Jordan - an unmanned desert field. All that's needed is a flat salt-pan somewhere - and given the huge number of possible landing grounds within the now-extended search area it'll take a while to locate - even from satellites.
One other thought - they wanted to crash onto a specific city or target - but ran out of fuel en route. Perhaps maths wasn't their strong point either.