I've actually always preferred a closed canon to an open one. That way I know the author can't mess it up any more than they already have. ;D
As for fandom, new people are discovering the pairing every day. Even in pre-Internet days fandoms were able to survive and even flourish long after any new material was made available. I think there may be a leveling off of activity, but HP fandom and Snarry will easily outlive me.
I think one thing that makes a crucial difference is participation. Nothing exists without loyal fans dedicated to maintaining it. I know Trek fandom has been around since the first show aired and you can still find the original fic today even though print runs were *very* small and rarely in a format designed to last. But it could have easily fazed out entirely if it weren't for fans dedicated to keeping the zines around and available in lending library format.
Sites and archives disappear overnight taking much of our history with it. Which makes it important that when we do move on that we make sure we pass the reigns to someone reliable and committed to maintaining the work. Though I suspect I won't be moving on anytime soon. ;D
Hm. A 'Slash Fest' sounds like fun. I know I've done 'so many days of something' posts on my personal journal before. They were always a lot of fun. I do know of one smallish Halloween fest and a few Samhain ones. But I'm not quite sure which fandoms or pairings those are for. I have too many fandoms. ;D