> nothing here is inconsistent with a mechanical problem
The total lack of communication combined with hours of continued flight with course changes is inconsistent with any plausible failure mechanism. I have seen no plausible theory of a mechanical failure that can account for these facts.
> Almost everyone believes it has crashed.
So? This would not be the first time that almost everyone was wrong.
> This seems like random speculation
It is certainly speculation, but it's not random. I'm a pilot with a degree in electrical engineering.
> When the NTSB/FAA say otherwise
"We cannot rule out X" and "it is probably not X" are not mutually exclusive statements. (And, BTW, an inside-job hijacking and a crash are also not mutually exclusive.)
We cannot rule out anything until we know for certain what did happen. But that does not prevent us from drawing tentative conclusions what is and is not likely based on what we do know.
False, nothing here is inconsistent with a mechanical problem. When the NTSB/FAA come out and say they've ruled out mechanical problems let me know.
> The fact that the plane has not been found is very strong evidence that it did not crash.
Almost everyone believes it has crashed. When the NTSB/FAA say otherwise let me know.
> A fire severe enough to knock out the comms would almost certainly have taken out the fly-by-wire system as well.
This seems like random speculation. Some of those systems would turned off by the flight crew intentionally as they go down a checklist.