Even as a tech problem, the new UI puts less information on the screen and makes it harder to interact with. It's worse by every observable metric, and I can't imagine they have successful A/B testing, because every other site on the internet that does a redesign (Facebook, Twitter, Youtube) commits to it, and Reddit has allowed users to stay on old reddit for like a year now.
The weirdest thing is that successful alternative reddit UIs exist, so all they would have to do is incorporate the features people are using alternative UIs for. They did virtually the opposite of that. This is exactly what Digg did, and the solution for Digg wasn't "allow people to use old Digg", it was "Everyone left and went to reddit".