I asked chatgpt what airspace class was associated with each market United had left. Looks correct to a quick perusal. I don't think they lose a lot of revenue leaving a Delta. The Charlie's you could argue about.

• Texarkana, Arkansas - Class D
• Flagstaff, Arizona - Class D
• Santa Rosa, California - Class C
• Stockton, California - Class C
• Alamosa, Colorado - Class E
• Pueblo, Colorado - Class D
• Destin-Fort Walton Beach, Florida - Class C
• Tallahassee, Florida - Class C
• Twin Falls, Idaho - Class D
• Springfield, Illinois - Class D
• Evansville, Indiana - Class D
• Paducah, Kentucky - Class D
• Alexandria, Louisiana - Class D
• Monroe, Louisiana - Class D
• Kalamazoo, Michigan - Class D
• Lansing, Michigan - Class D
• Muskegon, Michigan - Class D
• Rochester, Minnesota - Class D
• Cape Girardeau, Missouri - Class D
• Columbia, Missouri - Class D
• Fort Leonard Wood, Missouri - Class D
• Kearney, Nebraska - Class D
• Ogdensburg, New York - Class D
• Plattsburgh, New York - Class D
• Erie, Pennsylvania - Class D
• Pierre, South Dakota - Class D
• Watertown, South Dakota - Class D
• Abilene, Texas - Class D
• College Station, Texas - Class D
• Killeen, Texas - Class D
• San Angelo, Texas - Class D
• Shenandoah, Virginia - Class D
• Everett, Washington - Class D
• Clarksburg, West Virginia - Class D
• Lewisburg, West Virginia - Class D
• Eau Claire, Wisconsin - Class D
• Wausau, Wisconsin - Class D