I don't know the answer to your question *(I don't work on the game) but I think it's a good idea.
It would be nice to have competing airlines flying into the airport. You could make deals with them for slots, food services, fueling etc. (This, I'm pretty sure they will have).
Your airport has a given satisfaction rating, based on passenger satisfaction, and airport efficiency, combined with local economic factors that could influence demand at your airport.
After allowing an airline to grow, maybe at some sort of threshold for seats per year could be reached and you could get an announcement that says: X airline now considers your airport a small hub. A similar announcement could grow to medium, and large, and mega hubs.
I don't see how this would effect gameplay at all, except, it would be fun to know which category they consider your airport, and like in SimCity where there was village, town, megalopolis, and the music gets more ominous/big city sounding as the city grows, I hope Real Welders do something similar in Sky Haven, where as the airport gets bussier, the music becomes more frantic etc.. similarly in this game perhaps reaching certain passenger volumes is what would be needed to unlock larger control towers/ better terminal cladding/better gates etc, instead of having them all available at the start of the game, and money being the only limiting factor.
I'm kind of torn between passenger thresholds, or money in the bank being the limiting factor for upgrades to facilities though, as in real life, money is the limiting factor.
That said, maybe a combination of cost, and airport passenger thresholds would be more fun for the player.. Perhaps, players would need to have at least one airline consider you a small hub would be needed before unlocking say a glass jetway bridge or something similar.
If a particular airline was very impressed with your service and your rating was quite high with them, they would likely increase flight frequencies to the point that "small hub" status is reached. I think that could be quite fun.
I hope there is some sort of airline simulation in this game - I am not sure how deep they are getting with the different airlines that decide to service your airport, or what kind of simulation is behind them. I hope they try to mimic reality as much as possible and hope there is some depth to it.