I came up with a (barely) plausible incapacitation theory involving fire and deliberate cabin pressure dumping to fight said fire. Hi alt environment extinguishes fire but also extinguishes crew and pax. FBW airplanes flies on...

But my theory requires some coincidences (no radio call, fire takes out certain avionics, etc).

Much as I prefer to give the crew the BOD, Ocams Razor says it was hijacked. Logic then says it was hijacked by a crew member, because the "usual suspects" or their like would have done something more dramatic than vanish if they were in control of a fully-fueled 777.