So, why do the two cutout switches in the MAX work differently than before? The NG had one for disabling AP trim plus (?) SpeedTrim and one to cut all electric trim, meaning the thumb switches’ input too.
Max has both switches cutting all.
Why:
(1) Was there a perceived danger that one needs to make double sure to cut MCAS out otherwise it would fly the ac in the ground? Which it did anyway but for other reasons. But that switch choice shows that someone saw the danger.
(2) Troubleshooting what goes wrong with the stabilizer trim takes too long. Better cut out everything right away but keep the appearance of two switches to ensure continuity. That choice must then have been based on somebodies analyses that nobody uses the sequential cutout approach. Meaning AP and SpeedTrim never failed. That must have been documented.
(3) Disabling MCAS input together with the AP could not have been allowed due to certification issues. Having a switch for AP & SpeedTrim and one for thumb switches & MCAS was considered as too complicated, so two switches cutting out all seemed the best choice while keeping the layout.
It seem to me that whoever made that design choice was aware that MCAS could be dangerous and he/she tried a last minute fix.