Why do fashion people get trapped in a "look"? Like Anna Wintour and her bob, Karl Lagerfeld and his ponytail/sunglasses, and now Kate with her never-changing hair. It's a great haircut but she seems too young to be stuck in one look. Anna and Karl are older and set in their ways so it's more understandable they will settle on a trademark. I can't imagine wearing the same hairstyle year after year.
^well it has become her trademark now, and that might be why she sticks with it. or she simply prefers this cut and colour the most. i like it, even if it's true that it's starting to get a little repetitive- it adds something refreshing to her whole look.
She does change it a bit more than Anna does, it gets longer, shorter, and whatnot. Sometimes the sides are buzzed, and sometimes she just slicks them down, but they do all work around the same general shape/color. Plus, her hair is optic white. I can't imagine that growing that out long would be at all pleasant.