The problem is without MCAS and the AoA sensors that power it the 737MAX8 handles differently enough that it would be categorized as a different aircraft and require recertification of pilots. This happened because the 737 frame is relatively low to the ground and to increase fuel efficiency the only real option is a larger engine so Boeing pushed the engines up and forward. This changes the relation between the Center of Thrust and the Center of Mass on the plane causing it to have a tendency to pitch further up towards a stall when climbing so Boeing added the MCAS to make the new plane behave like the old plane by automatically performing the pitch down maneuver for the pilot.

This system is critical to the Max 8 remaining classified to 'fly the same' as the 50+ year old 737 air frame design so it's critical to pilots being certified to fly the plane at all. Ergo it is a critical system and needs more redundancy or the Max 8 needs to just have a new type certificate and pilots certified to fly it as well.

Boeing REALLY REALLY doesn't want to do this because it would require airlines to retrain their pilots to use it in their fleets making them less attractive to airlines. The whole fiasco is a confluence of 1) Cost of retraining pilots on a new air frame 2) need from airlines for a more efficient plane 3) the old 737 design not having any more room below the wing for a bigger engine (which is pretty much the main way we get more efficiency these days).