Since when is everything supposed to be "fair"? Life's not fair, it's full of billions of restrictions, and there's no reason the game should be completely "fair" either (there are dozens of explicit restrictions in this game that affect same airlines but not others, this is FAR from the only one).
A game must achieve some level of "balance" -- you cannot do that if you are, literally, treating every single player with the same exact "fairness" -- that'd work great if we all started at the exact same time, but the game, like real-life, is full of new entrants and old players. Ironically enough, restrictions are enacted in reality in order to try to make things "fair" -- tariffs, protections, air freedoms, regulations, etc -- all are applied to perhaps protect legacy carriers or perhaps enable superior competition depending on the intended purpose of the regulalory body. Here, in our case, the "regulatory body" is sami -- and it's his job to make things "balanced" which is often in direct contradiction with making things "fair".
If you want to take your chances at production slots opening up, you're free to do so. A number of players do this, some have managed to be lucky enough to be rather successful at it. You take your chances by placing orders for singular planes whenever windows open up, and you may, if you're lucky, get a lot more planes a lot sooner than if you just stuck a single big order in there. But you also take the risk that you won't find any open slots and the line only gets longer and longer and you never just get in it.
It's a gamble. And as with any gamble there's a risk but there's potentially a nice payoff too.