On the other hand, Reddit has tons of mostly-isolated subcommunities that will continue to thrive, because they aren't really dependent on the Reddit userbase; they're just the moral equivalent of hosted copies of the reddit codebase, like WordPress.com instances of WordPress.
I can imagine that people will stop thinking of "Reddit" as a website that everyone visits with "default subreddits" et al, but they will possibly continue to associate the name with the platform.
I agree, lots of communities that peak and decline, also continue forward perpetually - such as FARK, Digg or Metafilter - with smaller dedicated user bases. That's the most likely outcome for Reddit. It's so relatively inexpensive to operate, it's unlikely to fold. More likely it'll be swapped around in acquisitions, spin-offs, etc. as different parents try their hands at getting the mojo back (the MySpace / Digg scenario).
I can imagine that people will stop thinking of "Reddit" as a website that everyone visits with "default subreddits" et al, but they will possibly continue to associate the name with the platform.