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There is no need to add a third. The software update make MCAS to use both existing angle of attack sensors, and if they disagree MCAS will be disabled and a warning indication will be shown.

Then you risk a stall. If MCAS is non-essential why not remove it completely? And if it's essential, then you need >2 sensors to ensure it can operate all the time.

MCAS is not essential. It exist to make the airplane behave like the older B737.

This just isn't true. MCAS was added because it is a safety requirement that stick forces strictly increase. You don't want the thing to start stalling itself.

I think you mean decrease — pulling back should become less effective as you enter a stall.

Stick force doesn't refer to how effective any stick movement is.

Roughly speaking, stick force is the amount of force the pilot has to to apply to the stick to achieve a given movement of the control surfaces.

As the airplane gets closer to a stall, you want to make it harder for the pilot to pull it fully into the stall, by increasing the required stick force.

If the stick force decreased as the airplane approached a stall, then the pilot would have to actively push the stick back into more stable flight.

Not everything that is non-critical should be removed.

Autopilot? Not necessary, pilots can fly themselves. Still, it is added for convenience (and its consequences, less mental burden, more mental capacity to do other stuff).

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