The second sensor is on every plane, true, but the MCAS doesn't use it. The light just tells the pilot when the two sensors disagree. The criminally negligent

The second sensor is on every plane, true, but the MCAS doesn't use it. The light just tells the pilot when the two sensors disagree. The criminally negligent thing is having MCAS only use input from a single sensor, especially when there's a second sensor already there.