Isn't that the same for most games though, spikes around release then quickly fades? Achievements earned (for the games I've bothered to look at) would suggest that people don't actually play games much, but by the sales charts are still willing to buy them, even at the full price on release.
But there has been a snowballing post-release backlash over the things that didn't make it into the game. Shame, because the game isn't even bad. That's not just my opinion - the overall reviews reflect that as well. Mixed, yeah but none of them are outright negative. The hype really has hurt this game.
It would have faired a lot better if Sean had done no PR and Sony hadn't have put so much effort and persistence into marketing it. I don't think the reviews would have been any different, but there wouldn't have been this ridiculous hate campaign against the game and its devs either that's for sure.