Most space gear (excluding the stuff that gets you to and back from space) has been almost implausibly reliable. Voyager 1's thrusters still work far past their intended best-before date and they were not designed to have someone's life depend on them.
If a Manned Maneuvering Unit somehow failed, it would have been a highly extraordinary event. The crew on the shuttle along with their mission controllers could have made an assessment of the risk of failure of the second MMU.
This isn't like your computer kernel-panicking and you rebooting it and hoping for the best.
