I dont think there's a question of filling the aircraft. The question is what yields it returns. If 9 weekly is just enough capacity to meet demand then fares can creep up to stay profitable in the difficult winter months. If capacity goes up but demand needs to be pushed by weakening the fares, then yields fall and what was once a profitable route becomes unprofitable very quickly. Remember, the UK wasn't facing such high APD when LDY was 2xdaily from STN.