Yeah that's kind of intended. Machu Pichu and Washington are the last two airports. After that it's just a game of survival until the spawn rate becomes impossible.
I think this has potential to become a real game though, it's so mesmerising.
I don't even think that you would have to change much. The two plane types could have more distinct graphics, maybe prices speed balance. More vehicles maybe in general but unlock with time (like one type unlocked with every airport, in some form related to the location maybe.
