Interesting how Slash is declining while HN seems to have hit a plateau. I wonder if that means there's another site on the ascendency out there somewhere.
Subreddits have a lifespan, they grow to a certain point, eternal September, and then become husks of their former selves.
The defaults actually just keep growing. Their quality might have declined, but they are only ever more popular than before.
I'm not referring to the number of subscribers. That does increase but quality of posts and comments/discussions drops drastically once they reach a certain user count threshold.