Making good expansions and developing UI addons are totally apples and oranges; it’s kind of a ridiculous take to have.
The biggest problem they are going to have is not implementing them, it’s giving them the same customization options players are accustomed to having. I also just listened to an interview with Max from Liquid and some addon devs, and those devs said their impression was the team for Blizzard UI was abysmally small (like ~5). This will likely lead to problems with trying to maintain all the different addon features and getting them tweaked in a timely fashion.