Well, the fundamental structure of the game means that the game changes irreversibly (for all players) as the player base grows and airlines mature.
I think that, absent some kind of continuing expansion of the game's geography (i.e. the space game), the right way for the game to work is with periodic resets. Call them "Seasons"; this is Season 0, where we are still developing features and such, and future seasons would have a predefined length and some kind of final win condition. As time went on they would probably involve varying scenarios as well (like, say World War, where two blocs of countries can no longer fly routes to each other).