Perhaps the simple explanation is still the correct one. That's what one airline pilot thinks, anyway.
The left turn is the key here. Zaharie Ahmad Shah was a very experienced senior captain with 18,000 hours of flight time. We old pilots were drilled to know what is the closest airport of safe harbor while in cruise. Airports behind us, airports abeam us, and airports ahead of us. They’re always in our head. Always. If something happens, you don’t want to be thinking about what are you going to do–you already know what you are going to do. When I saw that left turn with a direct heading, I instinctively knew he was heading for an airport. He was taking a direct route to Palau Langkawi, a 13,000-foot airstrip with an approach over water and no obstacles. The captain did not turn back to Kuala Lampur because he knew he had 8,000-foot ridges to cross. He knew the terrain was friendlier toward Langkawi, which also was closer.
Take a look at this airport on Google Earth. The pilot did all the right things. He was confronted by some major event onboard that made him make an immediate turn to the closest, safest airport.
For me, the loss of transponders and communications makes perfect sense in a fire. And there most likely was an electrical fire. In the case of a fire, the first response is to pull the main busses and restore circuits one by one until you have isolated the bad one. If they pulled the busses, the plane would go silent. It probably was a serious event and the flight crew was occupied with controlling the plane and trying to fight the fire. Aviate, navigate, and lastly, communicate is the mantra in such situations.
What I think happened is the flight crew was overcome by smoke and the plane continued on the heading, probably on George (autopilot), until it ran out of fuel or the fire destroyed the control surfaces and it crashed. You will find it along that route–looking elsewhere is pointless.
Besides the obvious tragedy here, the one beneath it is that no one will ever be satisfied with the final explanation, even if they find the aircraft's remains and the black box somewhere at the bottom of the sea. Yet, the simplest explanations are often the right ones, and this one is quite similar to what other experienced pilots have told me. If there was a fire in the cockpit, oxygen masks would have been out of the question. If the crew was overcome by smoke, the flight was doomed.
For the sake of those families, I hope they find the crash site soon.