True - but in the video, from the back you will see K.'s hair reaches her shoulder blades. It seems to be shorter front, longer back. If she lopped 2-3 inches all around she'd still have quite a head of hair.
And just to show I'm not picking on Kate, I think quite a few royal ladies could do with a good trim: Mary, Marie, Maxima, and she who must not be named.
Mary in recent photos looks ok. Max definitely as her hair is fried from treatments and too long. I’d put MM of Norway on that list too. Marie ? I think a shorter style would be better. Vic should ditch the severe style with something shorter and softer. Mrs Spain should do the same. Mrs Cambridge although apparently having some hair sculpted needs to do more.