Everyone seems to be assuming a magic fire that can take out all the alternate communications - avoid the SATCOM and anything that prevents the aircraft flying normally - but depressurize the aircraft - and all before the hold fire warning and before the pilots notice and squawk or transmit Mayday.
There are competing sets of assumptions. :-)
The version of events in the media (lots of height, heading, and speed changes; avoids some radars; and then suddenly to make the data fit a straight flight to the southern ocean to finish on the same 40deg satellite contour as the beginning) is also hard to swallow.
I'll agree that the "pilots aren't the bad guys" scenarios require a catastrophe onboard that prevents communication.