The ventral fins (strakes) were not new on the MD-80. They were first fitted on the DC9-50, for stability at cruise. All DC-9 are subject to deep stall. The DC9-80 was fitted with a stick pusher after it became uncontrollable in a deep stall in flight test, and they had to deploy the parachute to recover.
AF447 cg was near the aft limit. It is unknown what would have triggered its apparent flat spin, but it may have not taken a large event in severe turbulence.
Maybe a spin chute could have saved AF447..
GB