Cows getting bigger
Some safety management thoughts. Below is a very basic and generic risk management matrix, something many of us will be familiar with and probably far more basic that the management system Boeing use.
Introduction of MCAS without additional mitigations increased the overall risk associated with an unsatisfactory trim runaway event. Boeing should have number crunched this one and it would be very interesting to see the numbers.
If you change the scale in the matrix from linear to exponential, and divide by a standard of 1 in 10^9 flight hours, that might be a starting approximation. The introduction of MCAS seems to have produced an additional risk of the order of 10^2, or 100 times what might be expected from a new and untested system. The mitigating factors noted in the emergency AD may have been assumed to reduce the risk by a factor of 10^4, but probably did not by a factor 10^2, under real world conditions and with actual pilots.