It depends on what part of the UI you are questioning. The file browser pretty much requires the double-click (because a single click is used to select what you want before clicking the CheckMark button). If everything were done with a single-click, there would be no way to open a folder to select individual files vs selecting the entire folder. If I remember correctly, I set up the file browser so a long-hold will act the same as a double-click.
The playlist could be changed to single-click with no problem. Same for the radio station lists (and HD1/HD2/HD3 selection).