I don't think the truncation thing is beneficial to the web. By the time the browser would terminate the connection, it has likely downloaded way more than it needs https://jakearchibald.com/2025/present-and-future-of-progres...
Imagine fast scroll across an image gallery on a slow connection (including cell handovers).
Or range requests, where a service worker only downloads the header+preview portion, and when clicking on the image, no need to re-download that.
Or even a browser that truncates all images, to protect users who might visit a page with huge background images that blows through their prepaid data plan.
JPEG XL anticipated, and accommodates, these use cases.