The spacecraft's already built with a (large) surplus of thrusters—the extra thrusters are all right there, pre-attached. It's because of that safety margin that NASA relaxed their requirements and launched with thrusters exhibiting high failure rates in testing.
Redundancies don't help you if you have a safety culture that treats redundancies as consumables. The more one team widens a safety margin in one place, the more another leans on that safety margin, and relaxes their own.
