The concept of the descent of AF447 troubles me.
Anyone who has brought a 208 ton aircraft from altitude (normal, normal, normal) knows how 'difficult' it is to do when heavy. The aircraft will VERY happily accel to MMax with very slight pitchover. From FL350 to surface is 'normally' 25 minutes (powered), clean, and with unrestricted clearance. Gliding, under control as described, 15-ish minutes. Emergency dive, spoilers, (loss of pressurization) maybe 10 minutes (Regs) to 10000, then who knows to surface.
Throw in CB activity, mod to extreme turbulence, nightime, surprise, inattention, one or more failed engines, reversion to some unknown control law, and how long will that take?
Who knows. After a high altitude upset. Geez, maybe speed of a rock from 7 miles up.
My point is that a LOT of attention needs to normally be paid to keep the speed under control in a descent.
These modern designs are so clean, they accel going downhill at the drop of a hat.