Just a few quick thoughts on how to improve long-term playability on unrestricted servers without getting pointlessly complicated:
The game uses a pretty sophisticated system to deal with the issues of old aircraft: They face greatly increased maintenance costs and lower image ratings, which both reduces profits and faces big challenges to address. Unfortunately, the rate of aircraft aging is extremely slow because it's 1:1 with the real life. If you are using freshly ordered aircraft, you are unlikely to ever run into those issues as they are so absurdly far away in time. You would never need to replace the aircraft because they would become unprofitable or unpopular over time.
However, the business in AS is modelled to allow for much faster growth than in real life. This means that at some time, you earn shedloads of money without ever having a need to spend it. Money just piles up endlessly so you just wait for newcomers to open up a new airline so you can crush them to the ground.
Imagine that aircraft would age much quicker, for example one full year in just a month. Aircraft, now set up at 24 years for write-off, would be written off after just two years. An airline which uses its aircraft for three real life years would need to face the issues of a 36 year old plane. Airlines would need to address the replacement of aircraft, and of course, the more aircraft they have, the more money they would need to burn in that process. New airlines could go two ways to attack large airlines: One, take the old and extremely cheap aircraft that large airlines dump on the used market. Two, begin with shiny fresh aircraft and compete against the more and more heavily deteriorating fleets of the opponents. Used aircraft not acquired by anyone after a certain amount of time could be taken from the market and be scrapped to prevent a flooding of the used market. This could also make the aircraft market more interesting for servers which do not have old aircraft types available.
One airline which does nothing for a long time would see its profits disappear due to exploding maintenance costs and decreasing bookings due to their worse and worse becoming image ratings. The constant need to replace aircraft would stop large airlines to pile up giant sums of money because they have to constantly replace their fleets. It wouldn't even be too tedious to do this, as flight plans can be easily transferred.
What do you think?