The good thing is that 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.
And, heh, that first year I read enough Snarry to put me in double digits. ;D I was just so thrilled to find all this great stuff. And it was all free for the taking. I'd have never dreamed that I would ever have the luxury to stop reading because there were typos or bottom!Snape. I was utterly floored.