It really is a balancing act. HTC could double the poly count of the planes and make them as realistic as anyone could, but the cost of this would be higher system requirements.
There really is no easy way to just dynamically allocate polys of a 3D model base on the system speed. It could be done, but it might take 10 minutes for a game to initialize, or longer, in order to reconstruct the models, realign textures and so on.
When you throw the network code into the frame loop of a 3D game, you have to give up quite a bit of processing power that could be used in the graphics engine. The more visible players/objects allowed in the game, the more network code has to do.
Balancing all the things a multi-player 3D flight sim has to do is more of an art than a science, if you want the game/sim to run on as many different type of systems as possible.
Comparing AH and IL2 is going to happen. I just thought I would lend a technical voice here

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