I guess it all depends on whether you're talking about general audiences or the hardcore fanbase.
I mean, technically, Dark Shadows fandom is still around and there's the occasional comic book or novel, but I'm not sure anyone would argue that Dark Shadows is really a going concern these days.
Nostalgia is not enough. Sometimes you need to reinvent characters and series for a new generation . . . like they did with Battlestar Galactica.