I just can't accept the idea that an onboard fire disabled everyone to the point they never made another radio call, yet the airplane flew for another 7 hours or so with a fire (that should have eventually melted the cockpit like with Swissair 111 or UPS 6). So that leaves the jihadist operator theory.
You cite two examples. I can give you more where no call was made. The fact is, one plausible scenario is crew theft, but just because something doesn’t make sense to a non pilot, does not make it any less or more plausible. The idea that something is “plausable” takes far more knowledge than the average online amateur.
Many airplane crashes have similarities, all are different in their circumstances.
Having said that, an onboard fire may not disable everyone or anyone, initially. However an exploding nose wheel tire can do alot of damage or one that "catches fire" .
The nose wheel tire is located very near what is called the electronic and equipment bay or as some refer, the E and E bay. An exploding tire, or slow burn could theoretically damage critical components knocking out electric power to communication equipment. And or damage other electronic equipment so as to create a high load on other equipment, causing electric busses to start disengaging.
A nose wheel tire can become overheated during the takeoff roll. It happens when a plane is heavily loaded, warm temperatures, long takeoff roll, late rotation, or rotation close to max tire speed. Not really uncommon.
In the scenario many are suspecting, is that smoke began to fill the cockpit. Oxygen masks are on and an immediate turn to the nearest airport is made. The climbing and descending can be because the autopilot has also become disengaged, that would explain why it took so much altitude to recover.
But as the 777 captain says, the oxygen system would have also become disabled or depleted and that is most unlikely. Very rarely to we find what is called dual failures.
The airplane would not necessarily be on fire enough to bring it down immediately. Small, fire, damaging equipment, producing smoke and might even go away if the oxygen source, (high altitude in an unpressurized bay) goes away and or fuel source is not enough to continue the fire.
Plausible but i am not buying it myself. But its the closest to Ocam razor that I have read.