The problem I see is the new game WILL attract a client base, most likely NOT AH players though Im sure some will find there way there and may even give up their sub at AH (making less players in AH to add to the problem of dwindling numbers). Once the new game starts getting players it will be easy for HTC to concentrate on adding "War Online: Europe", War Online: Eastern Front" and so on. This of course will take away development time for AH, after all they have a pretty small staff at HTC.
I guess I don't understand what you see as the alternative? Should HTC just keep doing exactly what they've been doing since 1999, and hope for magically different results? Will it be to anyone's advantage to see HTC's revenue continue to dwindle until they have to close the doors someday? The first priority for anyone who loves Aces High should be to ensure HTC stays in business. If HTC folded, you'd have no Aces High at all.
Do you really, honestly think there is some dar tweak, or hangar hardness setting, or new model Italian fighter that is going to turn the tide and suddenly put 650 players a night back into the arena?
Every product has a life cycle. If you are smart and lucky, your product life cycle curve has a nice long, fat tail that you can ride out for decades, but you may never achieve your peak again. Tastes change. Preferences for new models emerge. The competitive landscape evolves. That fat tail may not be enough, all by itself, to fund your company forever. So you add additional revenue streams to keep the lights on.
I think numbers can be stabilized. I think they can be maintained and possible improved somewhat. I think Aces High can outlast some of their near-model competitors and end up absorbing their refuges. I'm skeptical Aces High can ever get back to the numbers it had at it's peak, given changing tastes and other MMO competition.
I think Aces High is a great game. An important game. It's model appeals to a certain segment of the market. Maybe not as big a segment as in 2005, but there is a market that likes that open sandbox, persistent world, monthly subscription. I want to see it conserved for the future percentage of gamers that prefer that. HTC, as a company, may need to diversify so they can still provide that option to players for years to come.
However, as I said, there is so much overlap, that most work on any of those other games will also benefit Aces High. New development on those other games, will include new development for Aces High as a free bonus. And the revenue will keep the lights on for everybody.
You can focus on the possible negative outcomes, or focus on the possible opportunities. It's your choice.