Dozy, your comment on the diagnosis bit misses the mark, handsomely.
If they couldn't diagnose a stall from the information they were provided, then it leaves the question open.
The point of the AoA gage discussion over numerous AF 447 threads hinges upon how easy it is to see, on an AoA gage, if you are or are not stalled.
The other means of diagnosing pale in comparison when it comes to simplicity, though one must admit that a stall warning horn is usually a clue that one must pay serious attention to one's performance!
(I guess on this next bit, but I suspect that the pilots in the cockpit associated stall warning with Airspeed being unreliable, and for that reason ignored it as assumed to be spurious ... may be wrong on that).
That said, if it's easy enough to implement (and I'm pretty sure it is), that option should be exercised if for no other reason to provide more defence-in-depth than already exists.
While that is my feeling as well, I do appreciate the counter to any of us recommending that added indication:
the plane isn't supposed to be flown into stall, and pilots ought to be aware enough in their flying to take stall prevention steps when warning of impending stall (via scan or other alerts) tells them that they are about to stall. AoA would help that, but it is an additional scan item, as a supplemental scan, which takes us back to "if the scan is breaking down, or has broken down, what will the pilot look at next, and what will register?"
The requirements folks can make a decent case against. It takes something like AF 447 to demonstrate where that reasonable approach may still not account for how that gage helps when needed most.