The idea would be that airports can grow to contract depending on the service there. Every year or every month the total traffic in the airport would be calculated and the total number of passengers would then go up, down or stay the same. That way if airports go underserved compared to flights in real life the airport will shrink. If you created a hub at a particular airport that was quieter in real life there is a good chance you can grow demand there.
We never actually worked on the feature but when we created the world structure we fully intended on allowing functionality for it some time in the future.
