Just curious but we have a TON of aircraft types that are not available on this server. 707s, L1011s, older DC-8s?
Are we going to ever see these aircraft? There are other aircraft around the same time periods available but not these.
Maybe they are proven if available in RL and how many IF.
I believe the aircraft in the game are based on the real world airfleets at the time of a server’s launch.
I think it was mentioned somewhere that the amount of (older) used aircraft was intentionally reduced to a minimum to lower the benefits of "early bird" users. For example there are some aircraft, espacially at the Cargo sector like the AN-12, that give you a lot bang for your buck.
As I have seen some used 737-700 for sale appearing recently, there might be a chance that you’ll find some older used planes from time to time too. Plus the amount of produced aircraft and aircraft in use + on sale also indicate that there might be some used planes on hold at ATLO. But that is just me guessing here 
I know for a fact there are still Convair’s flying around. I saw one not too long ago hauling freight.