Is it possible the AC functional airport capacity could be upgraded in the future? My city is about 3 million people, and I've put all 3 AC airports and maxed out on capacity & growth stage. Realistically i think a single airport could handle more than 1 million people and it's rather too much for a city this size to have 3 airports. Maybe a stage 10 (1.5 mil) & 11 (2.25 mil) upgrade?