I had a similar question a couple of weeks back when I wanted to keep the header constant as the header had a hamburger menu which when clicked had a slide out menu. The feedback I had from usablity testing and reviews from other designers was :
- They found it annoying that they lost a part of of the screen space to the header always and had to limit their viewing to a smaller viewport than already expected.
- Another thing which came out strongly was that users dont mind scrolling up and down and had no objections to scrolling to the top to find the header.
- Another feedback I got was that since most users are accustomed to tapping the status bar to scrolling to the top, there was no need to keep the header constant (note this is only an iphone feature and might not be applicable across other platforms)
I also recommend reading this article about sticky menus. While the article doesn't explicitly touch on their use in mobile apps,it does raise one important point about how they can prove to be an hindrance
DISTRACTING AND INTRUSIVE
If not done carefully, sticky navigation can
be distracting. Some sticky elements get delayed when bouncing back
into position as the user scrolls down the page. Others are so tall or
wide that they dominate the layout and impede access to the content.
Navigation should be easily accessible but should not compete with the
content for attention.
Though your graph doesnt deal with navigation, it can end up being an annoyance as well since it would be constantly competing for attention with the rest of the content which the user is forced to scroll in a really small space
JohnGB answer about orientation issues with fixed positioning of content is also spot on.