I don't really believe that FH would be at risk. Sure, Aro'kai will probably have more graphics, from what I've seen, but people have fiends on FH who haven't even heard of Aro'kai. ((Met someone the other day who had no clue what me and another user were talkng about when she mention Aro'kai, and I asked her when it would come out, or if she knew.)) But, like Dark said, it depends on the people who play it and FH.
From my experience on gaming, there is actually a lot of lag behind more graphics, even some game derps, especially on multiplayers. ((IMVU being one, Second Life being another.)) So, I think many people who play Aro'kai might return to FH, once the traffic picks up in-game.
But, that won't stop me from trying it out. :3 I wanna play!