>It is vanishingly rare for users to scroll up when reading content unless they want to reach the top

This assumption is the problem. No, it is not rare for users to scroll up while reading. People are not perfect machines that read everything in one pass and understand it fully.

They may go back to re-read, or look at an earlier image or figure in the text, or otherwise. Sometimes people zone out for a minute and find they 'read' with their eyes but didn't actually take in the content. That requires going back.

For me, scrolling up to re-read is a basic use case of a web page. If it can't do that properly, it has failed.