So to answer your question there are now multiple issues at play and they are effectively redesigning the flight computer system as a whole. This is not a quick fix.
So it makes sense to step back, and fix things the right way before you reintroduce the system back into the market.
But what you cannot do is change the fact that the MCAS is necessary. The plane is always and forever going to be off-balanced and require an active system to account for that. Active systems will fail. Maybe they can fix it such that the failure rate is negligible, but I doubt it.
Unstable flight profiles are not foreign in aircraft, and requiring computer systems to maintain stable flight is nothing new.
The problem is entirely how this specific system was implemented... and that can be fixed.
They might be, but fly by wire does not imply that.
For the record, I don't believe Airbus plans are unstable in the way the MAX is, whose instability, from what I understand, came about because they were trying to fit larger engines on the original 737 frame (to prevent pilot retraining), and so had to move the wings higher.
Something like an F-16 however is less stable and requires constant adjustment of flaps etc. there dynamic properties of air to air dogfight are more important.
A quick google search seemed to imply most unstable airframes were military or experimental, not commercial. Unstable airframes are not appropriate for commercial airliners unless you can point to a good reason. "We don't want to train pilots" is a bad reason. The FAA should have pushed back very hard on this point.
Personally I think the flaws in the 737-MAX design process were so deep that it would be safest to just scrap the plane. It's not that the issues cannot be fixed, but rather that we'd be better off starting over from scratch knowing that good design principles will be followed the whole time.
At high thrust, slow speed, high aoa it is a little too close to neutrally statically stable.
C f. to airliners that can break up from dynamic yaw instability if the yaw dampers fail in some flight regimes.
"It highlighted the role of the co-pilot in stalling the aircraft while the flight computer was under alternate law at high altitude. This "simple but persistent" human error was given as the most direct cause of this accident"
I also acknowledge that the specific terms I use in this comment might not be 100% down to the letter terms used in the aviation industry (such as unstable frame). The point being made here is corners were cut, Boeing is 100% to blame for the situation they find themselves in with the 737 MAX.
In avionics, fighter jets are unstable requiring computers and algorithms to keep them flying. This is in part to the speed of which they travel, the [Dassault Rafale](https://en.m.wikipedia.org/wiki/Dassault_Rafale) has multiple levels of redundancy to correct negative flight conditions (it has three computers I believe).
Even with the fixes, the plane is still going to be unstable. However, fortunately, unstable planes like passenger jets which move slower can still be manually flown (unlike fighter jets moving at Mach 1). But, I am definitely not going to fly on a 737 MAX if I can avoid it, not until it has proven itself in service for 10 years. I wouldn't say fixing an engineering problem with the software and additional sensors inspires confidence.
Dynamically unstable airframes should not be a thing in passenger jets. The only reason the 737 MAX is unstable is that they put bigger engines on a 737 and moved them up because of how low the 737 sits on the ground, thus disrupting the centre of gravity and requiring pitch correction when at full thrust.
The Airbus A320 has no such issues with its design. It can safely glide during total engine loss for quite a long distance. Could the 737 MAX claim the same thing?
At full thrust the nose would point too far upward with the new design, potentially causing a stall. MCAS automatically pushed the nose down if the nose was detected to be pointing too far up. Pilots received a two-hour training course on an iPad before being allowed to fly on the 737 MAX, they didn't even highlight the new MCAS system. The training material didn't even mention the new MCAS software...
The worrying thing about this entire situation is the MCAS system relied on a single AOA (Angle of Attack) sensor and overreacted and took control of the plane. Given planes are known for their levels of redundancy, a single sensor and some overzealous software coupled with other mistakes resulted in hundreds of lives lost. Pilots were not even notified in the cockpit that the MCAS software had taken over control of the plane. How insane is that?
This all happened because Boeing pretended the 737 MAX was basically the same as the 737. The cause of the problems was not because of poor software, it was because Boeing tried to cut costs and put in a bandaid solution for the unstable frame they created. The software did its job, it's the lack of redundancy and ability to manually override the system that was the problem. They're just doubling down on it because there is no alternative to the engineering problem they created.
MCAS = May Crash Any Second
That coupled with lack of training of how to react to exactly that scenario, and we have the events that transpired.
All of which is fixable one way or another.