Aces high might be the best quality sim out there but its no longer relevant. Newer games have filled the gaps with planes and vehicles that were only prototypes and tiny production runs. People like being able to play with things that you don't get a chance to is the central draw of any game.
I'll assume that by 'game' you mean, specifically, online MMO air combat sims. This thread is full of theories, philosophies and such expressed as fact, almost as if the posters took a survey across games, platforms and communities or even intensively played all the games to learn what attracts or loses players and how. From Fugi's 'what this game needs is more cowbell/behavioral coding' to your 'what makes a game special is access to every single plane designed to successfully or unsuccessfully pursue combat in the skies from any nation, pre-war and post war included.'
An example of the latter could be:
Notice that in this free to play sim plane may be bought individually. This is one of the cheaper ones at ten bucks. And what a winner of an experience it would be (theoretically, in my own untested opinion). That may vary from player to player as they spend x number of hours pretending to be a pilot in this crate while other player pilots shoot it down. Is it accurately modeled or was it enhanced to stand a chance?
Then there's:
Notice that this airplane is more expensive to buy. $46.00 bucks more (3 months of flying lots of great aircraft in AHIII).
But it does come as a bundle:
Note the 'crew training' and, better yet, the 'unique aircraft that do not need to be researched' thingies. What, they make it up as they go? Well, I wonder how stringently they research the not so unique aircraft players must individually purchase.
And this is supposed to be the draw? Well, that and they never heard of or tried AHIII.