I know the community is getting smaller, but Smurfing and Kicking unskilled from games is probably the LEAST issues that caused the downfall of ICO. Minidumps and a variety of other problems decided ICO's fate along time ago. Frankly, Developers made a game that was unfit for the online community for a very long time and that destroyed its online credibility.
Today's low player counts are primarily the result of the problems Sins had early on and its low player counts. I bought the game in early March, about two or three weeks after it was released, and the most people I ever saw online at one time was 280. That might seem like a lot considering the current player counts, but that's a tiny number for a hot new game. If a game only has 280 people online one or two months after it's release, it's probably going to be doomed to have much lower player counts years down the road. Sins online multiplayer suffered from numerous technical difficulties until Entrenchment was released, and at that point the fate of the online multiplayer player counts was pretty much sealed.
In contrast, if Sins had had 5000 people online at peak times, that would have been enough to sustain a larger community for a couple years. Even if the peak player count dropped 95% between early 2008 and today, that would still give us a peak player count of 250 today.
In summary, I think that today's low player counts are primarily the result of early technical problems and the game's purchaser player base being composed overwhelmingly of people who are predisposed to playing single player offline. The introduction of Impulse probably didn't help matters--there must be Sins players out there who are still on vanilla Sins v1.02 or v1.05 who have never visited the forums nor heard of Impulse wondering what the Ironclad Online button is supposed to do. (If it were for Multiplayer, wouldn't it be the Multiplayer button?) They've probably concluded that it's just a broken feature.
