I think software developers reading this thread, and people who have suddenly decided they could make software developers, should understand that far, far better software developers than you thought long and hard, and worked on this system design, and developed it in the light of experience, and have produced a system that has worked fine for all the world for 25 years.........except for 1 flight.
Rainboe, if you look closely, I think it is not the software developers or system designers but those who think they could do it better ("how hard could it be") who are suggesting the system should be changed.
As you say, anyone who has done system or software design for a fault tolerant system knows just how hard it can be to diagnose faults reliably. It's easy enough to say that a couple lines of code would have fixed this specific situation. But it is much harder to spot all the situations that wouldn't be fixed and would be created by the change.