As long as people are playing and enjoying dominion, they will want to talk about it. You just need to look at the game reports forum for a while to see that even strong players are seeking help and opinions on their losses and showing off exciting and interesting games. The game is too varied, too deep and too surprising for conversation about it to simply cease.
On top of that the community seems really strong, helpful and inclusive. I've only been here a month, but have spent the time amazed by how great the atmosphere is here compared to many other online forums I've posted on. I feel free to post my opinions and questions, and many of the members here seem patient, helpful and just as curious about my problems as I am. It seems unlikely that the community will die just because there aren't expansions coming out every six to twelve months.
I think the bigger question is what will happen when Isotropic goes down. I've not really played there, but I wonder how many people will be switching to Goko when Iso goes down. Will those people that stop playing online dominion just fade away from the forum?