The pass the buck circle jerk is how this design flaw came to exist. Everyone in the engineering organization needs to have the balls to point out systems design errors. Management needs to listen to them and not issue "make it work" marching orders. Regulators need to not delegate their responsibility to the previous.
More than one person could have put their foot down and demanded triple redundancy. That this didn't happen suggests even more safety concerns lurk in all of Boeing's products.