Good post, and thanks for the heads-up about fetch limitations.
fetch is indeed low-level. In fact, I’d argue that its purpose is to be a low-level replacement for XMLHttpRequest rather than eliminate the need for libraries. For example, fetch is heavily used in ServiceWorkers, which obviously need support for low-level, explicit functionality.
That being said, I agree that the standard body might have come up with some extensions to fetch to make it more usable for applications. Hopefully that might still happen. I see nothing in the extensions you would like to have that mandates breaking backward compatibility.