Given how far they're willing to go with dark patterns to con people into installing an app I'm actually amazed that they still bother to maintain this legacy versions. Seems like it would've been easy to let them bitrot into being unusable.
According to my traffic stats (moderating ~400k subscribers), old + mobile web frontends make up about twice the traffic of new frontend. Apps are by far the most popular, about ~3x of all web frontends combined.
I do not know a single person that moderates a 100k+ subreddit using the redesign, everyone still seems to be using old + modtools + enhancement suite. Personally, I believe this is the only reason they didn't kill it already.
Edit to add: The new frontend is the default for logged-out users, and it still makes up half of old. If that isn't a failed redesign project, I don't know what is.