The fixed header is actually pretty stupid when you think about it. More and more people are using 11" and 13" laptops, meaning that a fixed header takes up space that could be used for actual content. In some cases the size of the header means that users on small screen have to scroll endlessly.
It would make more sense to go back to having a left or right menu, if the goal is to utilise screen space better.
Fixed headers also break pressing space to page through a document - my usual way of reading the web. Space advances the viewport by one screen-height, but with a fixed header, not all of this screen-height is visible.
It would make more sense to go back to having a left or right menu, if the goal is to utilise screen space better.