If only Airwaysim worked like in the real world. But it doesn't. Here you have 35 years maximum age for a plane, and it's usually unprofitable with regards to maintenance before that. Airwaysim also calculates maintenance purely on age, while in the real world it's a mix of cycles (landings), flight hours, and to a certain degree, age.
So in the real world, you can fly a 40 year old aircraft when it's needed, you only pay for the maintenance depending on how much you use the plane. But here, you pay for it regardless. Which is ridiculus.