I am convinced that she purposely keeps the same hairstyle for historical reasons. When people think of her a hundred years from now they will all think of the same image, not the one with her hair in a bun, or a ponytail, or a pixie, etc. Also, she doesn’t want to fall prey to fads, like everyone else does. I also think that Princess Anne follows her example. She’s had that same school librarian bun for 50 years! Even when she was only 20 years old she looked old.
You could be right.
Thanks for the input.