If they have 2 AoA's and they disagree, they can indicate that with the light AoA disagree, disable MCAS and display on the CDU that MCAS is disabled. So the pilot knows what's happening and take corrective action.
Except that now we have a plane that behaves differently compared to the plane the pilot actually is trained for (the whole reason to implement MCAS in the first place). So now the plane comes down because it stalls where it should not according to the training of the pilot. Gee, makes me confident to fly...
I understand this as if you need to pull with 10KG of force to get a 10 degree angle of attack, then you need to pull with at least 20kg to get to a 20 degree angle of attack. I'm not going to claim that's 100% accurate but I believe it's close enough.
This is a generic requirement of part 25. So had the 737 Max been a clean sheet design it would have been a requirement. It's not just a compatibility layer to make it behave the same as old aircraft. It's a fix to make it meet the requirements of certification.
Training only came into it at the point that Boeing declined to add it to the difference training, charitably, because they felt the system was covered by existing procedures or, less charitably, because they wanted to downplay any differences to make the aircraft more appealing.
But that still means that if you disengage MCAS, you have a plane that flies out of certification specs (and by extension, out of the training), then?
To phrase it another way, without MCAS the 737 Max didn't meant the requirements of certification and so wouldn't have gone into production. The alternative wasn't to train pilots into how to handle the new stall characteristics.
Of course, the region of flight that MCAS was intended to activate in is also a bit of an edge case. Without MCAS we may have seen an aircraft flown into a stall, most likely around a critical phase of flight (landing or a go-around), but that would have involved a pilot ignoring numerous warnings (stick shaker activation, 'stall stall' oral alerts etc) and you'd be better categorised as pilot error than directly related to the handling characteristics.
Unfortunately, due to a terrible system design, MCAS appears to be activating in regions of flight outside of the design intent and with greater authority than it should ever have been given.
[1] https://www.737ng.co.uk/B_NG-Flight_Controls.pdf page 8.