It's been said that engineering is the art of risk mitigation. Not augmentation. Not elimination. Because there are an infinite number of ways things can go wrong, but only one or at best a handful of ways they can go right, when things fail they often do so in ways and sequences and combinations never anticipated. That's why systems are designed to be robust. To handle the unknowns. Or to try to.
That high-altitude high-speed ejection system failed for one pilot. But it worked for the other. It didn't work perfectly in either instance. But it worked well enough in one instance that a life was saved. And that's a successful mitigation. Just ask those Apollo 13 guys 46 months later.