I always assumed that the new reddit UI was more for app based usage than browser. And there is/was a hope to steer more people towards using the app.

I also assume they are smart enough to realize that some people will simply never use the app. In which case they're not realizing as much benefit from forcing the new UI on them. Sure, there's a cost to maintain the old one. But as long as things aren't breaking it's easier to keep it.

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.