Yep, people will play both, or just one. Doesn't much matter anyway.
It'll lose some people, yeah, but not all of them - and unfortunately, not all of the trolls and other riffraff will go, either. Some of them may move to AK, but I have no doubt that some will stay.
Myself, I'll probably play just AK for a day or two, get bored and leave it, unless there's something in it that I end up really, really enjoying. It happened to me with FH - once the new of the game was gone, it was just a chat room to me, which was really dull. So, I stopped playing the game. I still keep up to date on this community though - if a big update happens and the game becomes enjoyable, I'll start playing again. Until then, I have no game preference, although I do play IT.
I don't even like to compare IT to FH though - think about it.. IT grew much better over the years, just as I'm sure FH will. I played it the first year it was released, when it was just a chat room, and made lots of friends who've stayed with me during that time (Noana, Buri, Koori, Rizu, Telly, and Jay the Human.. just to name the ones that I've remained close with over the years <3). I knew quite a few people, lost some friends, gained some friends, got involved in drama.. but it was still a great community. When it was closed, I was sad and tried my best to hang on to the friends I'd gotten. When FH was released, I was excited to meet more people, but soon found out that the community was nothing like Impressive Title.. most of it was full of people who annoyed me, or who just weren't interesting. If you wanted to get to know someone you saw, you'd go over and more often than not, be ignored. That's why I left - no one who I ever saw playing the game seemed friendly to me.
So, maybe the community getting smaller would be kind of good when AK is released - it may bring the community closer together and with a tightly knit group of people, it's easier to make friends and most people who cause trouble aren't a problem for very long. I dunno, I'm just ranting I suppose.