People are suggesting that a fire couldn't have disabled the communications in the sequence in which we are being told they were and presume they did.
I've had two on board fires in aircraft. Both of them included the popping of circuit breakers on multiple panels disabling multiple items on different buses which made troubleshooting nearly impossible. One of them disabled lighting in the cockpit and instrument lighting at night and disabled two radios which I had been trying to use but never knew they weren't working. (thank you McD kapton wire flash overs)
Since the B777 has automatic fault detection and logic, a fire could have led to the automatic shutdown, shedding, or isolation of different electrical systems.
Anything is a possibility right now and until we get those boxes or salvage the airplane, we won't know.