While it's a huge task, at some point there will likely be a graphics engine overhaul for AH. Who's to say when it will be?
Online games are, almost by definition, products that evolve. Developers count on ongoing subscriptions rather that one-time boxed sales, so there has to be constant development in order to keep player interest piqued.
At some point in the future, that might well mean an all new graphics engine (AH 2.0?). As it is now, AH has (relatively recently) begun supporting the DirectX8 API, so there are quite a few tricks in that bag that could be exploited long before the current engine is exhausted. Also note that some of the artwork from the original release (cockpits and skin textures) is gradually getting updated to be more detailed.
Cheers,
phaetn