Think about it like the Antilock brakes on your car. Suppose the wheel position sensor fails. It's fine if the car puts up a warning light and says that you don't have antilock brakes anymore. You can drive fine without them until you can get them fixed with a minor safety impact. It's not fine if the wheel position sensor fails and this causes the car to slam on the brakes going 65mph down the highway.
You wouldn't hear about it unless the activation was triggered by egregious pilot error and you're scouring aviation news sites.
It's not fine if the wheel position sensor fails and this causes the car to slam on the brakes going 65mph down the highway.
Been there, done that. It's an unpleasant failure mode, but it is survivable.
And, anecdotally, I've had ABS kick in in some occurrences for which I was very thankful.
Suppose the crash probability on a normal flight is 1/1E7, but without MCAS it's 100x more dangerous, or 1/1E5. Suppose MCAS failure probability is 1/1E6, the probability of an additional crash due to the failure of MCAS is 1/1E11, which is acceptable.
The problem is that in practice, the crash probability if MCAS fails is empirically 2/3 instead of 1/1E5, because MCAS actually causes the crash rather than merely failing to prevent a crash.
"ABS has close to a zero net effect on fatal crash involvements."
Further down, there is a chart which seems to show that ABS is associated with a huge increase in fatal accidents in inclement weather, and a large increase in side impact accidents, both fatal and non-fatal.
Maybe there's something obvious I'm missing?
This is something all 737 pilots are trained for, so they have to balance that pitch up from the engines throttling up by pitching the plane down more than they would otherwise need to. The precise relationship between throttling up and pitching up was changed when the 737 was modified to create the 737 MAX. Namely, on the 737 MAX the pitch-up is more extreme. It is not so extreme that it makes the airplane a bad design, but it is extreme enough that pilots need to not expect the legacy 737 behavior.
Had pilots been trained for the performance characteristics of the 737 MAX specifically, it would have been perfectly fine. But they weren't, and instead MCAS was meant to paper over the difference so pilots could be kept in the dark (which is cheaper but borderline homicidal...)
To put this another way, in some hypothetical universe where the 737 MAX was the first 737 ever, it would have been introduced without MCAS and pilots would have said it handles like a dream. Then, when the 737 NG was introduced after the 737 MAX, there might have been a reverse-MCAS system implemented to make the NG handle like a MAX. That reverse-MCAS system may have then failed catastrophically.
The problem with the 737 Max is that the engine nacelles are further forward and larger than previously, so the lift that the nacelles make at high angle of attack is greater. This results in less stick force being necessary to maintain a high angle of attack than at lower angles of attack. This is uncertifiable behavior. Hence, MCAS, to augment the manuvering characteristics of the 737, to increase stick forces at high angles of attack.
In a hypothetical universe where the Max was the first 737, it'd be fly by wire and all the stick forces would be synthetically generated anyway.