I am in the middle of my first long term game (#1) and from what I see I have to ask... for me it will be a challenge to pay for the next generation of aircraft....these big mega airlines (600-800 planes or more) with not a lot of money (800M - 1.5B) and older fleets how will they survive based on past experiences?
Unless you can switch to more modern jets, they will die at some point.
Either MX (maintenance) will kill them, because the costs grow exponentially with a/c age.
Or Fuel will kill them, because fuel prices go really high in the mid to late 2000s (usually, depends on Sami's settings though).
Or plane age will do the job. Once an aircraft is 35yrs old, it will no longer fly, and you lose the slots on the routes after a short while longer.
Truth be told, I once started up using 20 year old 727's from the lease market and ditched them prior to C check where I just replaced them with another one from the brokers. The planes were doing great and earned me millions while their leases were under 250K per month. This approach allowed me to get factory new stuff while expanding the airline. And I didn't went down and kept the 727's in the airline for over 3 years.