My .02Cents....
1. Catastrophic power failure
2. They had no idea what their altitude was...not flying vfr.
3. They augered in at flight speed...accounts for the debris field and box damage.
You can buy a general aviation battery powered device for about $3000 with gps altitude and position, artificial horizon, weather, speed, etc. It also stores airport data. There is enough information on it to fly the airplane under instrument conditions after catastrophic power failures. You don’t need power to determine altitude (air pressure), anyway. It would be really surprising to me if commercial airplanes don’t have similar devices. If nothing else, they can use it until they deploy auxiliary power devices.