The major issue is liability and responsibility. No concept in history ever survived freeing a thinking being carried by something of the responsibility to intervene misbehavior. Even further, in all proven concepts a "higher skilled" being always has the responsibility to react in case of malfunctions - be it machine or men, example: if somebody with ATPL is on right seat and something happens due to a PPL malfunction, every judge will ask nasty questions, or 2nd example: if somebody with a Royal Yacht Master is on board of a boat and something happens, the same judge is going to ask the same nasty questions. As long as there is no principal decision to accept a mechanical/electronic device as intellectual superior to mankind, this issue will not be solved. Add: and if it is solved that way, it is no longer my world. Pax carriage solved that problem by locked doors to the cockpit, but after Germanwings this ain't look like a viable option.