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.