People like to bring up the instability of high performance jets, but I'd like to point out that they generally have ejection seats and don't carry passengers.
Airbus jets carry passengers and have been fly-by-wire for decades. Cables and hydraulics have been replaced with software and computers. Like it or not, the software and hardware that make up the flight computer are an integral part of the airframe on most newly produced passenger jets, regardless of who makes them.
It's an interesting point. One of the reversion modes on Airbus is "direct law". Under that control mode the stick displacement is proportional to the control movements.
If, hypothetically, an Airbus had a problem in the same region of flight would it even be detected in flight testing? As far as I'm aware it wouldn't be as direct mode is a reversionary mode intended to get the aircraft safely on the ground.
One of the things that is regulated is stick control forces.
They cannot be greater than a set level (essentially, an average strength pilot), particularly when performing critical manuevers.
This was actually the whole reason MCAS was engineered in the first place: to lower the effective stick force required to within the acceptable limits in certain scenarios.
So presumably Airbus in direct mode could still have similar issues flagged in direct mode, if the plane behaved in such a way as to require unacceptably high stick force to move control elements (even if it was a 1:1 mapping).
Technically I'd say they introduced MCAS to increase the control force in certain scenarios (high power high angle of attack). But yes I mostly agree with you.
One thing I'm not sure about is whether Airbus would have to demonstrate proper controllability (i.e. adherence to control force regulations) in all phases of flight and corner conditions.
You could have a scenario where by they have a complete control reversal on the approach to stall but under normal law the pilot would be oblivious. This would obviously show up in direct law.
I'd hope they would have to demonstrate both things: (1) that a given mode can or cannot be active in a certain scenario, and (2) how the plane behaves in all tested scenarios under all modes potentially active.
Is there a reason this necessarily wouldn't be the case?
My guess is that it's considered so unlikely that they wouldn't be required to demonstrate the full range of behaviour. In the same way as I wouldn't expect them to demonstrate all of the edge cases of the flight envelope with a failed yaw dampener. I could well be wrong though, I haven't found any good references.
Fly-by-wire is not about airframe stability. High-performance jets are fly-by-wire because high-performance generally means you need to build an unstable airframe, and computer control is the only way to compensate for it.
But that's not commerical aviation - that's combat aviation. That's "you fly faster or the missile catches you and you die anyway" aviation.
Building intentionally unstable airframes for civilian aviation is a very different proposition. And unrelated to the use of fly-by-wire.