The moment was sadly ironic. On Wednesday, as city leaders celebrated the renovation of Revolution Mill — a former Cone Mills textile factory that now houses shops, offices, restaurants and apartments — another Greensboro textile mill was closing its doors. White Oak was the last Cone textile mill operating in Greensboro, the last to make iconic American denim in the town where it began, the last still serving its original purpose. Although the timing was purely a coincidence, it served to underscore how important historic preservation and new-market tax credits are in promoting economic development in towns where traditional businesses and industries have withered away. There are few sights as disheartening as shuttered factories that once provided a livelihood for hundreds of workers. But that doesn’t have to be the end of the story.