A smart airline would flag few people that do this and put them in a high-touch queue where the airline confirms if they intend to stay on for the final leg of

A smart airline would flag few people that do this and put them in a high-touch queue where the airline confirms if they intend to stay on for the final leg of the flight. If no, the airline modifies the passenger's flight details and makes it an A-B flight, and the seat on the B-C flight is marked available for purchase (maybe at a discount last-minute type price). If Yes, nothing changes. This way the airline can still sell that ticket. Savvy customer is happy with high-touch customer service, airline makes more money.