Users aren't equivalent though. IIRC, "the vast vast VAST majority of people" don't comment or post, either.
It's absolutely fascinating that they have both versions. To me it illustrates so perfectly what the Internet was vs. what is has become. There's probably more money to be made from the infinite scrolling and dark patterns, but it's also completely unusable and comments and discussions seems to be of less concern. Just keep scrolling.
Basically the 90% / 9% / 1% split between people reading, people interacting and people creating. I would be very surprised if the 1% contributing and creating on reddit wouldn't be using old reddit for the most part.
It's also sad because even though the old reddit is available, it's not the same. Since all the users are using the new UI, you get a totally different type of user engagement than you used to get I feel. Nowadays it's rare to have any sense of community or long discussion and every subreddit has moved along the spectrum to shallower interactions, image posts, and less distinctive character.
They still run i.reddit.com, which probably only has a tiny fraction even old.reddit.coms users. Also, I'm pretty sure that quite a few power users still use the old site and they probably want to keep them in. Someone has to buy gold and repost all the content, after all ;-)