What your saying is an extremely valid assumption and I agree with it to the most part.
But I however, think that with the number of gamers that come on gaia so high it will eventually thrive on it's own. While yes, there will be MASS chaos trying to check out the MMO the first few months once that dies down I believe there will still be a thriving community in the MMO. I don't think it will be quite "mini-" as you say.
Gaia is mainly Forum based but once the MMO comes out many peopl will spend more time in it then in forums.
Also, with the news of an MMORPG out Gaia will attract new users mainly interested in that.