I agree with your first sentence - accidents are usually caused by small problems compounded over time.
In this case, however, I can't imagine a situation where a small fire causes enough damage to render all communication equipment useless, incapacitates the crew/passengers, but doesn't cause the plane to crash for another 6 hours.
I don't like conspiracy theories because they require many people to keep quiet (people aren't very good at that). But in this case we are left with many scenarios that range from improbable to nearly impossible.
I read a case recently where there was a small fire which initially wasn't noticed, it was only when someone was checking the voice recorder after the accident that they realised the electrical noise on the recording was caused by the fire which allowed them to trace the initial source of the fire (retro fitted wiring that wasn't padded well enough)