Make room for the 2000 lb Gorilla in the room. 
A lack of increasing subscriptions may be a recruiting issue.
A decline in current subscriptions is a game issue.
While Aces High has been and continues to be the best game of the genre out there, HTC has done very little to keep the game "fresh" to hold the interest of a lot of the "veterans" of the game. Of the 6-7 of the people that I personally keep in touch with that flew this game, I believe I was the last to cancel my subscription. The one common underlying theme for all of us appears to be lack of interest. While some also cited finances, family, time and equipment, almost all also included the "SSDD" defense as well.
HT has stated in the past that ppl losing interest in a game is a fact of life and there is nothing that can be done about it. I fear this is the accepted thought related to this issue. As an outsider looking in, it seems to me that they tend to form an opinion on a certain subject and don't revisit it after that. If that is the case, it's a shame in my opinion. Having said that, they do have to be careful about changing too much to maintain the vets interest and end up irritating the majority or making the game too difficult for beginners.
Also, I suspect HTC may consider vets to be bad for gameplay as a whole. The vet that actually helps new players consistently is rare. Most vets pwn the noobs which leads to frustration on their part and makes it difficult for them to get into the game. Not to mention, there are many vets out there that are downright hateful to new players. For this reason, I think HTC may secretly want the vets gone bc afterall, they are a very small percentage of the playerbase that has a mostly negative effect on the other customers. I don't completely subscribe to this thought myself, but it is one of the better explanations that I can contrive that would explain what seems to be a lack of interest in retaining vets.