And more plane types will not be added if it does not appear profitable to do so. Catch 22.
When HTC first released AH, they had faith in the business model because of Warbirds. They don't have the same with WW1. I'm not clear how involved HT/Pyro were with the initial release of DoA, it was released about the time iMOL stepped in, but they did not have the opportunity to see it grow. From what I hear DoA was successful for a while. There's a bunch of former DoA fliers who play RoF, and probably even more who play OFF. What I am convinced of, however, is that you can't build a WW1 arena on the model of WW2. The aircraft and ground vehicle abilities just aren't there. This was a war that stagnated for lack of offensive weaponry, after all. WW1 needs its own concept in order to be viable.