I like that it's information dense. Posts don't take up a lot of space, comment chains are easier to track with the boxes, it's more obvious which posts will expand in window and which will go to a new link. I don't care if it's pretty, I find it far more functional.
Tbf, when people say they love old.reddit, they very well could be referring to some of the better-looking themes. A lot of old.reddit subs, for example, use Naut. Some also use Minimaluminiumalism, or Simplit.
It’s got to be two separable layers. Sounds like the lemmy server just happens to be bundled with a particular UI layer that can access the server’s APIs.
But you could switch out the whole lemmy ui with wefwef if you wanted to. It probably doesn't have all the admin functionality, but theoretically it's possible.