I wouldn't say that you can't correctly diagnose the failed AOA sensor in the majority of cases, but the conditionals to disable a safety system are probably go

I wouldn't say that you can't correctly diagnose the failed AOA sensor in the majority of cases, but the conditionals to disable a safety system are probably going to have to be more rigorous than "AOA changing between extremes without significant change in pitch, airspeed or altitude". Just for starters, imagine you've hit a particularly violent updraft and then a downdraft... Though I'm not sure if such weather conditions would exist anywhere you'd plan to fly an airliner. (And I'm kind of being "perfect is the enemy of good" here, sorry. Any "AOA sensor is wonky, disable MCAS" methodology would probably have resulted in better outcomes than the one they used. It's just that none of them are really perfect)