I would argue that the most important factor when considering old reddit vs new reddit UI/UX isn't a matter of preference based upon performance, certain content, or habit. Old.reddit is actually just better for the end user experience overall and new reddit UI is better for Reddit's ad revenue.
Many times a user not wanting to switch to a new UI isn't based completely in effort/adaptability but a history of experience with product life-cycles weighing more towards business objectives over time. e.g. Facebook calls users lazy for not trying out "improvements" and blame old soccer moms for being inflexible when they're just trying to extract more money. Not that businesses spending effort to get more money doesn't make sense, because it does, but businesses love to lie about this common user complaint.
The fact that new reddit defaults to showing only a few comments on the post, followed by recommending 20 other unrelated posts, just shows how badly aligned that design is with their goals.
Reddit is a glorified web forum. Period. Making comments hidden and difficult to browse basically negates 50% of it's function (the other being media + content discoverability).
I imagine it's quite well aligned with their goals of getting increased user engagement metrics from increased clicks to read stuff from casual browsers to the site, and convincing regular users they should download the app
Of course it's extremely badly aligned with their regular user's goals of reading comments, but that's solved by using the old.reddit urls if not the app, whilst the casual browser coming in from Google or a link gets the full on contempt for users' desire to actually read threads UX until they've bumped the user engagement metrics up by clicking on more stuff.
Many times a user not wanting to switch to a new UI isn't based completely in effort/adaptability but a history of experience with product life-cycles weighing more towards business objectives over time. e.g. Facebook calls users lazy for not trying out "improvements" and blame old soccer moms for being inflexible when they're just trying to extract more money. Not that businesses spending effort to get more money doesn't make sense, because it does, but businesses love to lie about this common user complaint.