JD-EE
Consider that the Auto Pilot dropped as a result of its being "unable" to control the a/c with its set of limited control commands.
Remanded to "Manual", the pilots have a serious challenge, again as the result of what I propose is the a/p's "long leash". Given that the a/c is oscillating inside an envelope of prescribed parameters, and its cruise (.82 Mach) speed, whatever caused the ap to retreat is now in the hands of a man whom we assume is at Cruise, SA-wise. A hyper-vigilant Chief Pilot would be challenged as he never had been in his life.
Now consider another possibility. The a/c was not under threat at all.
As the micro-crystalline Water Ice packs up the pitots and hobbles the OAT sensor, the a/p "reads" the flight status as dynamic, rather than static. It doesn't think, and it can't hear, so it makes inputs to "correct" what is becoming a rapidly deteriorating (to it) situation. It is likely reading slowing a/s, so lowers the nose a titch, and perhaps augments thrust, just to "smooth and speed" the "recovery". But it isn't working. It's only plan is to correct, not stop and assess. Sometimes doing nothing is better than doing something. But the autopilot soldiers on. How quickly did the limits cause the a/p to shut down? Let's hope it didn't allow the Pitch and speed to get too wack, but one thinks it did.
Energy. At a high velocity, "slowing down" takes time. The a/c is clean, and the descent devices are not available for quite awhile, they would be battered to bits. What caused this a/c to slow rapidly? If, as mm43 proposes, the a/c was in a turn and Impacted not far from its "onset" position, there are only unpleasant suppositions.
Small and sharp things leave first, wingtips, tail bits, skin that is disturbed by the departure of objects of ever increasing size, which exacerbates the deceleration and donation of parts to the air and sea below. At some point the velocity is low enough and the relative strength is high enough that a "balance" is achieved, and the airframe settles into its likely flat spin to the Sea.
Nothing about this is proposed to be certain, there is too much missing from the puzzle. It is minimally informed supposition. Guesswork.
587 lost an engine (as in separation) almost immediately, wake turbulence is a beast, low and slow. Primary failure at altitude and close to the speed of sound? Somehow (someway) 447 recovered to impact the Sea on the verge of complete recovery?? Not saying it's impossible, but....
So that is one answer to this aircraft's having been "swatted from the sky". I tend to agree that velocity was lost quickly, and disastrously.
bear