Even the smallest of airlines can bring down larger rivals’ fares for dozens of routes across the country. According to AAA, this week will mark the busiest Thanksgiving for air travel in history , with more than 5.8 million passengers flying domestically . Only a small percentage of them are booked on Spirit, the troubled airline that filed for Chapter 11 bankruptcy protection last week . Yet millions of others — make that almost all of us — benefit from Spirit’s presence and should give