A machine that relies on sensors has two ways to detect when a sensor has failed: another sensor, or human observer input.
I don't know how avionics hardware engineers do it, but in this neighborhood of the Internet, we don't trust inputs, and especially human user inputs. Because every unverified, unsanitized input is an attack vector for bringing down our software and the system it runs on.
From what I have seen, the MCAS in the crashed planes relies on a single sensor--the AOA vane in the nose--and was almost solely responsible for catastrophic loss of altitude. This model of passenger jet has a paid upgrade option to add a second sensor, with disagreement detection.
My question is why don't the yoke inputs from the pilots count as disagreement with the AOA sensor? If the yoke is consistently counteracting the action of the MCAS, why can't it disable itself automatically and illuminate a light to indicate it has failed?
I'm guessing the pilots would have more time to search through manuals in-flight to clear the fault and re-enable the system than they would trying to disable it while it's stubbornly trying to crash the plane due to a single point of failure.
It's not hard to adopt the defensive mindset that your users (or your professional testers) are maliciously trying to destroy your beautiful program with a combination of stupidity and cleverly designed unanticipated inputs. When hardware gets involved, one can personify Entropy as a being that is trying to destroy everything you love and kill you.
How would Entropy take down a plane and kill all passengers? How about it freezes the AOA sensor in the "nose is at +90 degrees pitch" position? How do we defend against that attack vector? Pilot training? Oops! Entropy also made them forget that page out of thousands of possible pages of procedural training during the critical seconds they needed to remember it. The only way to fight Entropy is by making random events more independent, rather than causally linked in a failure cascade.
I don't think this course towards blaming Boeing's lack of documentation and/or pilot training is helpful. I don't think there's any option for Boeing but to immediately recall and retrofit all aircraft to the multiple AOA-sensor option, at their expense, and refund every airline that actually paid extra for it.