Most fandoms with closed canon slow down and at some point (after a decade or more in some cases) they're pretty much dead. But I've noticed that less with book!fandoms and more with show!fandoms. Books seem to have a more timeless quality to them, I'm not sure why. It's easier to jump on the bandwagon even if the story was published 50 odd years ago.
I've always thought of HP as the book version of the Star Trek and Star Wars franchises. Actually, I kinda wish it were MORE like ST, because that would mean we'd get more canon fodder every decade or so. Not necessarily in the same 'verse or time, but that''d make it even more interesting IMO.
Still, we have movies and a theme park and maybe some day there'll be a animated movie (like Final Fantasy!) where the actors don't age too fast, LOOK the part and have facial expressions that make sense for various scenes *lives in hope* With stuff like that I can definitely see a resurgence for the fandom.
But even without it, with the number of fests and the size of the active community, we still have a lot of steam to carry us forward over the years. There might be a time where it feels like "been there, done that" but as long as new blood keeps coming in, there's always the possiby of finding something fresh in fandom, if not canon, that makes us fall in love with our ships and characters again.
And while peeps may get interested in other fandoms, there's no reason why they can't be active in all of them to varying degrees. I'm pro multi-fandoms 'cause it prevents you from getting burnt out from a single fandom - you get to try creative and fabulous fanworks with totally different ideas and then come back for some HP!love when the urge strikes again. At some point, maybe you *will* loose interest in HP, but I usually find that a single gorgeous fanart/fanwork can rekindle waning interest in a ship.