In the real world (game try to be the more real possible i think) we have anti-monopoly laws. Maybe you need to implement something as this. As an example, put one limit in the quantity of orders for new aircrafts one airline can do, based in one percentage of your actual number of aircrafts. This open space for others too. This successful airline can always get used aircrafts (available in greater number than actually) but could have problems with the competition from airlines with new-modern aircrafts. In the real world, too, producers of any type donīt accept normally to have a big portion of your production in the hands of one client. They prefer to have a greater number os clients.
So why don't you go to EK and tell them that no, unfortunately, they can't order 90 Airbus (when they had already reduced their need from 120 to 90 because of space constraint at DXB)?
I'm sorry, but it doesn't work like this in real life.
The problem is that this time the used a/c market is very "stingy" and there are too many players around. 400 are more than enough, in my opinion, especially now that we have the multiple bases feat!