I've always assumed there's some PM sitting there at Reddit, watching the usage metrics every month, and as soon as it dips below his threshold, he'll order it shut down. "Should we tell 1 out of 1000 users to fuck off, or 1 out of 100 users?"
I think they have stats showing that the most prolific contributors all use the old reddit interface and so they don't yet want to risk their more engaging content.
