Are you playing Begginers World for sure? I have no idea how you can have so many aircraft in Poland? 
GW2
Every possible line within 1100NM and more than 25 demand from the 6 big airports spammed with F27s. Plus many further ones covered with B727s. I had an opponent that covered everything, and I had to follow him everywhere. It was rather bloody. Then he made a mistake on LH flights (you shall not build LH flights from Poland, period), and I ended up alone, oversupplying most routes.
A newcomer there would have tough times to breathe.
The problem is that income difference is very visible for me... There are other companies fighting with me of course and if the plane with the best routes has a c-check I see immediately huge difference in my incomes. However, after 1 day of this "switching" I'm already tired and don't enjoy the game like before. And I really want to do things beter, stop with earning $17.000 - $20.000 after tax when the weekly income is $85.000 - $90.000. 
Everyone suffers from C & D checks. That's fair. Your opponents suffer from them, you suffer from them, I do(though I make enough money not even to notice it). Very early in the game, it may be worth to have new arriving airframes switching with the old one, so that the best routes(i.e. the first ones created). but as soon as the old airframe is back from check, do fly it at all costs on new lines. Those babies are too costly to keep them in reserves.