> The primary use case is to not load images you may not ever see because you may not scroll that far down the page and they don't appear in the viewport.

That is the "bandwith saving" part. It's user hostile because it is another non-JS privacy leak and because it means that you can no longer open a website to read later when you might not have internet access.

> obviously hero images and other images important to the initial user experience

Hero images are typically the most pointless and irrelevant to the user experience. If you want to save bandwidth then push back against this fad.