That's not how redundancy works. Optimally, a system would be able to recognize any significant discrepancy between the two sensors and throw an error message instead of crashing a plane. This just took the broken sensor reading and fed it to the control system. 

The sensors also don't just report "problem vs. no problem," they report an angle of attack and the system uses that to correct. the difference between a sensor reading that makes the plane nosedive and the readout from a functional sensor in a commercial flight profile is distinct.