but trying to stall any aeroplane while on base leg to see what happen does not sounds to me to be really a technical issue.
Well that would depend on perspective. One of the problems we have is that modern technology makes aircraft so reliable that human behaviour takes over and we naturally lower our guard towards the unthinkable. "It won't happen to me". Nine times out of ten they probably would have got away with the manoeuvre at that altitude and thought nothing more of it.
So there are two possible paths to take with this investigation:
1) it was crew error full stop. Had they been flying higher this perhaps would not have happened, end of investigation and wait for the next similar accident or
2) expand the investigation and potentially uncover failings or mistakes or oversight within the maintenance environment. Were there issues with 2 out of 3 AOA sensors? If yes what happened?
Were correctly trained staff employed?
Has the company a proper quality system? etc. etc. etc.
Now depending on the answers to question 2 this scenario may well end up incorporating technical issues as well.
There are other departments involved in the safety equation and nobody can state with 100% certainty that this accident would not still have occurred at 20,000 or 30,000 feet. We don't know.