As you already know, AC credits are the primary currency in AirlineCommander and there's probably no one the can say that have enough of them to get everything that they want. Something like this would be possible if that person was using AirlineCommander cheats but let's assume for a moment that they are not.
AC credits are the resource that are locking the most interesting, the coolest and absolutely the most useful upgrades that we can get for our plane. Although not necessary for advancement, these AC credits play an incredibly important role in this game and can easily make you skip large portions of waiting, grinding and doing all sorts of unnecessary things simply because you choose to stay and stick with the free to play approach.
This is all okay, the game needs to make some kind of money after all but, after a while it can totally kill your desire to continue playing the game so it eventually works against itself because if somebody doesn't want to buy credits, they never well and will instead get more and more frustrated with the game unless they completely give up on it.