Well to add to that part on 5 forces

During Professor's Zou lesson today he mentioned that the profit margins of the airline industry will remain low because of the strong competition in the airline market. This was reiterated in a video with Michael Porter himself who agreed that the airline industry characterises an industry of strong comeptition where there are strong substitutes with different airlines, strong buyers, strong sellers and relatively low barriers to entry. 

Michael Porter also mentioned in the interview that an industry where there is low competition, high profit margin and hence low 5 forces in the market is the soft drinks industry.