The obvious answer is yes, of course Haven is sustainable. Gameplay can continue forever, and it is up to the players to provide meaning and "fun", for themselves and for others.
But lets evaluate some serious facts:
1. After the first few months, every serious faction has every resource, and the only they left for them to do is mass pearls, gold, and constantly repeat the same actions to raise quality ad infinitum. This isn't nearly as much "fun" as the starting out.
2. A core component to Haven is the concept of loss and interaction between players. Game play mechanics as they are make interacting with other people less efficient and more dangerous than keeping things in a "closed circuit." Once people get resources they lock them down, and only interact with other people with alts unless they are involved in some mostly meaningless exercise of combat designed just to "keel your enemies" without serious repercussion for either side (with the sole exception of the rare siege that actually does real damage.) Personal interaction *is* important to the META game of Haven. But that all happens on Skype, and is involved primarily with dealing with your allies and gaining intel about your enemies. You never need to (nor should you to be most effective) use your character to interact with others in the actual game.
3. Player retention is poor! Moving on from gameplay facts, the majority of people that end up playing Haven don't keep playing long term. Many of the long term players also only play in spurts, because they have been everywhere and seen most everything, and there is little else for them to do. Is there something we can do to make the game more fun in the long term?
I don't think this discussion is pointless. If we are going to have this discussion, it might as well be now, when the developers are working on Hafen and what we, the community, suggest may ultimately give them a few ideas as to how to remedy some of these problems.