I don't think the hidden cities are going to cost the airline a lot of money. After all, if I walked out on the street right now, and polled people "what is a hidden city in the context of airline tickets," how many people would even know about it? Let alone know how to do it, or have the guts to try it. I don't know that it is correct to say that promo fares are going to disappear because people exploit the hidden city loophole. Plus when you miss your first connection, it often deletes the rest of the flights from your reservation. I wonder what will happen, some day, when a pax is denied boarding for say coupon 4 on the hidden city, and then the pax sues the airline. Then I guess the courts will really have to take a more definitive position on the whole matter.