The problem is that a few crashed Boeings could lead to massive distrust of airline travel across the globe. Is that a larger long-term economic drain than grounding Boeings for a year? I'm not even sure how you'd begin to model that. Heck, I'm not sure we collectively know how to evaluate the risk of any given Boeing falling apart mid-air. It's probably still very, very low, but it might not take much to put people off flying completely.