Spirit Airlines was once a potent force in the U.S. aviation industry. Its demise will reveal how strong that influence had been in recent years when air travel had already begun moving away from the low-fare model that Spirit pioneered. The airline’s shutdown on Saturday after years of financial troubles resulted in the loss of 17,000 part-time and full-time jobs, and disrupted the plans of tens of thousands of travelers. But aviation experts say it is not entirely clear whether Spirit’s absence will have a significant, long-term impact on the industry, travelers or the U.S. economy. Airlines will probably have an easier time raising fares and many will absorb Spirit’s gates, check-in counters and other assets at airports in the New York area, Las Vegas, Fort Lauderdale, Florida, and elsewhere. But the effect may not be huge, aviation experts said, because Spirit had shrunk a lot recently and was in its second bankruptcy in two years.…