When reading through the aircraft deliveries thread, I realized that we need to consider a deeper problem first in order to have any "realistic" delivery/backlog system work for AE. I think the root cause of the ridiculously high demand for aircraft lies in the fact that AE gameplay is mostly about constant expansion. Think about it: what do you do when you play AE? Especially for the large airlines, most of the playing time is spent adding new routes and assigning aircraft to them. If we want to have realistically sized airlines and number of aircraft in service, it would involve shifting the bulk of gameplay away from constant expansion. However, what we shift that playing time to cannot be some tedious, repetitive chore like scheduling maintenance for each aircraft or going down a huge list and lowering ticket prices one route at a time.
So if* we really want to get rid of the perpetual expansion aspect of AE, we'll have to make the airline maintenance and improvement aspects more substantial but also fun. Maybe aircraft scheduling doesn't have to be a tedious chore. Or exploring and expanding into new markets should be a more involved process than just magically seeing the exact supply/demand and plopping some planes down. Or we should make the product aspect of an airline more important and dynamic, and include opinions from passengers evaluating their choices between you and your competitors. With more realistic (read: thinner) profit margins, you would also be affected by and forced to respond to dynamic aspects of the world like spikes in fuel prices. In summary, we would need to add substance to parts of the game other than the "open new route" button.
* I know some people like AE for its simple, fast expansion and don't want to get bogged down in other details of their airline. We do have the option of keeping the gameplay the way it is and accept the unrealistically high passenger/aircraft volume as a side effect. It is a game after all, and making it less fun to play for the sake of realism may not be a worthwhile tradeoff.