The problem is not whether or not everything should be added. I'm sure HTC would love to see every aircraft that saw service in the game, just like the rest of us.
The problem is, that that is an unrealistic goal. The harsh reality of things is that the game is created & run by a company with a staff of what... 7 people? And, beleive it or not, aircraft take time to model, especially when you consider the tiny staff of HTC, compared to a larger company like EA or something. So it's not so much what aircraft should be modeled- as they will hopefully all be in game 200-300 years from now- but, in what order they should be added, from most important to least.